Lethe - Episode One has 19 achievements:

• 3 story achievements
  
• 5 miscellaneous achievements
  
• 7 "collectible" achievements
  
• 3 repeating task achievements
  
• 1 idle achievement

It's possible to do everything in a single playthrough. The game doesn't have any form of level select, so anything missed will require a new game.
If going for a single playthrough, Markitrap! will require reloading a checkpoint. Read that particular achievement for more details.

These three will unlock simply by playing through the game.
I have the Power!
You have gained Telekinesis!
Unlocks at the beginning of level 4.

Motherly Love
Witness the Doctor's death by the hands of Mother.
Unlocks towards the end of level 8.

Step into the Void
Finish the game.
Unlocks at the end of level 8.

These are only possible at specific points in a level.
What a view!
Step onto the docks and look at the sea.
Level 1 - Walk out onto the end of pier at the very beginning of the game.

Resourceful
Break the loot crate at the end of the first level by jumping on it.
Level 1 - When you reach the radio transmitter, climb up the nearby ladder and drop onto the crate marked "Fragile".

Fly, you fools!
Fall into at least one abyss.
Level 3 - First possible in level 3. After surviving your first encounter with the Doctor and climbing a ladder, you'll reach a large open cavern. Run forward and jump into the abyss. If missed here, there are plenty of other opportunities to get this in the game.

Refreshed
Use a toilet to replenish your health.
Level 3 - First possible in level 3. After dropping down the mine shaft for the first time, follow sign to the Stairs, then the Restroom sign.
Level 4 - There's another bathroom in the Sleeping Quarters if you miss the opportunity in level 3.

To be or not to be
Grab the skull in the Laboratory basement with Telekinesis.
Level 5 - You'll eventually need to replace a black tank with a yellow one to unlock a door to proceed through the level. Next to the room where the yellow tank is found there's a skull on display. Pick it up with telekenesis for the achievement.

I use the term collectible loosely since some of these aren't actually collected. But regardless they're tasks you perform every level.

Lightbringer
Turn on all of the lanterns in a single level.
I recommend doing this on the first or second level as these are the smallest and there are no enemies. Lanterns you can use are highlighted in blue and difficult to miss if you keep an eye out for them.

Markitrap!
Step onto at least three traps.
Usually this would conflict with Escape Artist, which requires you to disarm all the traps in the game. If you are trying to complete 100% in a single playthrough, you will need to step on a trap, restart the checkpoint, repeat twice more for the achievement, then finally restart once more to disarm it.

Can't fool me!
Disarm least one trap.
Self-explanatory. The first trap in the game is on exiting the beach house.

Escape Artist
Disarm all traps in the game.
There are 15 traps in the game. However, this achievement unlocked for me at the beginning of level 8 even through the 15th trap is later on in level 8. Regardless, I've listed all 15 traps in the guide below.

Bookworm
Read all the notes in a single level.
See Librarian.

Librarian
Read all the notes in the game.
Self-explanatory. There's a counter in the bottom left corner of the screen that tracks how many notes you've read in the level.

First-Class Explorer
Explore every nook and cranny.
There are 32 locations in the game that count towards this achievement. These spots usually hide notes or recovery items. Simply visiting the spots are enough for it to count. There's a counter for these on your achievements page, but the counter may be slow to update.

Hulk Smash!
Break 10 objects.
It would be nigh on impossible to miss this during regular gameplay. Jars break by simply running into them, loot crates can be broken for recovery items, and there are specific things that need to be broken to progress in the game.

Firestarter
Set fire to at least three objects.
See Burn baby! Burn!.

Burn baby! Burn!
Set fire to at least five objects.
Level 6 - This can be easily done at the start of the level. You start next to an open flame and there are chairs, crates and barrels in the next room that can all be burnt. Simply pick them up with telekenesis and move them into the flame.

Just Chillin'
Spend 60 minutes in a single level.
The counter resets on starting a new level, and only saves on autosave or at a radio transmitter. You can simply load up any save and stand in a spot for an hour.

Below is a list of all the tasks required for the missable achievements listed in order.

Few things to note:

• Notes: You can track how many notes you've read with the counter in the lower left corner, but there is no way to track how many you've read across the entire playthrough.
  
• Traps: There is no way to track how many traps you've disarmed.
  
• Explorer: The progress for the number of locations you've visited can be tracked on your achievements page. This counter is slow to update though, so it's recommended you check constantly and maybe even wait for it to update correctly before moving on to the next level.

Achievement: Lightbringer
It's easiest to unlock Lightbringer for turning on all the lanterns in a level in this level or the next. There are 8 lanterns in this level:

• 1 on the beach
  
• 3 in the house
  
• 3 in the mine area
  
• 1 in the house with the valves

Note 1/9
From where you wake up, look in the open suitcase at your feet to find the first note.

Note 2/9; Explorer 1/32
Walk along the beach to the giant pipe leading out of the cliff face to the beach. The note is on a table under the pipe.

Achievement: What a view!
Walk to the end of the pier and look out to sea.

Note 3/9
Enter the beach house and pick up the note on the table in the first room.

Note 4/9
In the kitchen is another note.

Note 5/9; Explorer 2/32
Exit the kitchen and walk down the hall. Enter the bedroom on the left and read the note between the beds.

Note 6/9
In the study adjoining the bedroom is another note.

Trap 1/2
Exit the house through the back door, and on the ground immediately in front of you is a trap.

Note 7/9
Climb up the nearby ladder to find the note by several levers.

Note 8/9
Use the button/lever to lower a platform. Jump across and drop down the other side. Walk forward to find a fenced in area on your left which leads to the mines. The note is stuck on the fence at its entrance.

Trap 2/2
Enter the house across from the fenced area. The trap is on the floor in the middle of the room - by the oil drum with candles.

Note 9/9
Look on a desk in the building as Trap #2 to find the last note of the level.

Turn both valves in the building with Note #9 and return to the fenced area. Pull the lever next to the fenced area to start the mine cart. With the mine entrance clear, enter the mines.

Achievement: Resourceful
Walk through the mines until you find the radio transmitter. Climb up the ladder and drop down on the "Fragile" marked crate to break it.

Save at the radio transmitter if you want, the door beyond the transmitter ends the level.

Note 1/5
Walk down the mines until you reach a room with three locked doors. Find the note nailed to the wall by the door on the right.

Trap 1/2
Climb up the ladder and cross the plank. Crawl through the hole and drop down to find the first trap in front of the lever.

Achievement: Markitrap!
Since this trap is close to the start of the level, you could purposely trigger it three times for the achievement. Just make sure to restart the checkpoint again and disarm it.

Note 2/5; Explorer 3/32
Pull the lever behind Trap #1. In the mines area beyond, go straight at the first intersection then left at the next one to a room with the note.

Note 3/5
Return to the first intersection and go down the long hall to another open area. Find a hole in the floor and drop through. In the floor below, unlock the door with the lever and the note will be in the next room on your right.

Trap 2/2
The lever here activates the elevator to end the level, but it won't work until you switch another lever. Go through the door to an an earlier room and the door by Note #1 is now unlocked. Follow the linear path beyond. After raising the portcullis with the lever and going through the hazy room with mushrooms, the trap will be in the next room in front of the lever.

Note 4/5
On a table next to the lever above.

Note 5/5
Pull the lever and leave the hazy mushroom room through the other exit. Collect your first health supplement and continue down the hall to a radio transmitter. Next to it is the last note in the level.

Exit the room and follow the path to loop back to the first area of the level.

Achievement: Lightbringer
This is probably the easiest level to unlock Lightbringer if you don't have it yet. Unlike any other level, you can backtrack through the whole level until you find all the lanterns.

Make sure you've read all the notes in the level (especially if you reset for Markitrap!. Return to the lever that activates the elevator and take it to end the level.

Follow the linear path until you find a hole in the wall. Crawl through to find your path forward blocked by flames. Walk up the stairs on the left side of the room to three valves. Turn each one until the screen shakes (that's the best way I can describe it). As you return down the stairs, the Doctor will be approaching you. The game tells you to sneak, but it's easier to just rush to the where the flames were and climb up the ladder to safety.

Achievement: Fly, you fools!
You'll exit in a large cavern. Walk forward and jump down into the dark abyss.

Note 1/7
From the top of the ladder, look left to see the note under some canned food.

Note 2/7; Explorer 4/32
Return to the abyss and jump over the rocks and pipes to the door. Follow the sewer, under the steam that appears and into the next room. Across from you is a door marked "Restricted Area". Inside is the note.

Note 3/7
Return to the previous and climb over the crates to reach a radio transmitter. The note is on a cabinet nearby.

Open the large doors, jump over the broken fence and drop down the pipes to the walkway below.
Jump across to the other side and open the door to the next area.

Achievement: Refreshed
Follow the signs to the stairs, then to the restroom. Use it.

Note 4/7
From the restroom, open the gate to reach the Surface Access stairs. Enter the red tinted area beside the stairs, press the switch to open the door and crouch under to reach the note.

Return to the room with the Surface Access stairs and through the only door you haven't accessed. Continue through the rooms until you reach the hall and a fence blocking your way. Climb on the oil drum and crates nearby and over the fence.

Explorer 5/32
On the other side of the fence, enter the door immediately on your right for some food.

Note 5/7
Continue down the rocky hallway to reach a vent. After crawling through the vent, look left to see the note on top of a button.

Press the button and take the elevator down. Open the next door then crawl under another to reach the long shaft again. Climb over the railing and drop to the bottom. Follow the large pipe to the end. Crawl through a smaller pipe then follow the linear path until you reach a ladder, a locked door and the doctor spawns beyond the locked door.

Trap 1/2
Climb up the ladder to reach the second radio transmitter, and in the next room you will see crates marked "Lethaum". Crawl through the gap in the crates and the trap will be by the ladder.

Explorer 6/32
Climb up the ladder and find the hole in the floor to drop down a level. The doctor will spawn here. Follow the outside wall of this area to a dead end with food and a health supplement.

Explorer 7/32
Still in the area with the Doctor, find the arrow on the floor that points to a hole that goes under the floorboards. Under the floor, instead of climbing up the first ladder, crawl right between two mushrooms to another ladder. Climb up to more food and a health supplement.

Note 6/7
Climb up the ladder you passed earlier to proceed. Further on, lower the crane with the lever and jump down. After falling into the water, climb out and walk along the pipes to reach another radio transmitter. Next to it is a note.

Note 7/7
From the radio transmitter in Note #6, follow the long hallway to two rooms. The one on the right has the note next to a body on the floor.

Explorer 8/32
Head into the next room from Note #7 and enjoy the sight and sounds.

Trap 2/2
Enter the other room in the long hallway and crawl through the duct. In the room at the end is a fenced in area with a trap on the floor.

Pull the lever to turn off the fan you saw earlier in the duct. Crawl past the fan, then drop into the mine cart at the end to finish the level.

Remove the oil drum blocking the door with your new power, and further on pull the lever from through the gate with it as well.

Note 1/6; Explorer 9/32
Go right from the first Energy Supplement and enter the Archives room. Inside is the note on the desk.

Note 2/6
On the floor right of Note #1 is another note.

Note 3/6; Explorer 10/32
Exit the Archives room and turn the valve across the room with telekinesis to fill the pool. Cross the pool then continue along the linear path until you reach a room with a strange device in the middle. Before fiddling with the device, enter the Control Room on the right and inside is a note with the radio transmitter.

Note 4/6
Exit the Control Room and in the opposite corner is the note is on a shelf.

By the shelf is a yellow tank hidden in the corner. Pick this up with telekinesis and replace the black one on the device in the middle of the room to restore power to the switch used to exit the room.

Explorer 11/32
Before crossing the water in the next area, walk up the stairs to the Emergency Exit door. Throw the barrels up here into the water below.

Note 5/6; Explorer 12/32
Cross the water by jumping on the barrels, climb up the scaffolding on the other side and through the duct. Take a left in the room you exit at and crawl through the hole in the wall. At the back of the next room is another note.

Return to the where you exited the duct and take the other path. A blue field appears which blocks your telekinesis. The exit to the level is on the other side of the room, but you can't pull a lever to open the door without telekinesis.

Enter the room on the left and use telekinesis to turn a valve in the room beyond.

Note 6/6; Explorer 13/32
Enter the other room in the blue field now (right from where you came in) and then left at the next intersection. Crawl through the hole in the wall and find the note by a sink.

Return to the intersection and go the other way. Knock down a metal sheet through the fence with telekinesis and turn the valve. Go back to the blue field to find its gone now but the Doctor will be coming towards you. Hide in the area with Note #6.

After the Doctor passes, return to the central room, and open the door with telekinesis. Open Elevator Access and take the elevator to end the level.

Note 1/15
Enter the room marked Laboratory near where you start. On a table on your right is the note.

Use the oxygen valves to turn off the steam in this room and remove the floor panels to access the floor vent. Crawl through, climb up and walk around to a lever. Go through the now-unlocked door near where you climbed up to be back in the first room of the level.

Explorer 14/32
The door next to you is unlocked now, but flames are blocking the path. Go back to the Laboratory and redirect the oxygen again to stop the flames. A few rooms further on, climb up the stack of crates and over the fence. Open the gate at the end of the hall you drop into to find a health supplement hidden behind some crates.

Note 2/15
Go through the door near the gate. Enter the Archives room, the note is on the desk.

Note 3/15
Also in the Archives room, the note is in the cabinet with the gramophone on top.

Note 4/15
Exit Archives and enter Laboratory 1 opposite. The note is on the left as you enter.

Note 5/15
On the cabinet opposite the entrance to the the Laboratory 1.

Note 6/15
Enter the short hallway through the other door in the Laboratory 1. The note is on a cabinet near the swinging light.

Note 7/15
Break the planks blocking the entrance to the other Laboratory in the hallway. On a table in the Laboratory is the note.

Note 8/15
Remove the floor panels in Laboratory 2 to access the floor vent. Exit the room you climb out of and proceed forward to find the Sleeping Quarters. As you enter, the note is on a table on your right.

Explorer 15/32
Explore the bathroom in the Sleeping Quarters.

Note 9/15
From the hallway with the entrance to the Sleeping Quarters, walk down the hall to the next room. As you enter, the note is nailed to the wall on your right.

Note 10/15; Explorer 16/32
On your left while looking at the previous note are two doors. Enter the left one and find the note nailed to the wall. This room is a kitchen with stoves and food.

Note 11/15
Go down the stairs next to the large valve in the area with Note #9. In the room at the bottom is the note nailed to the wall above a lever.

Explorer 17/32
Use the lever to open the trap door. Jump down, straight at the intersection and explore the dead ends in that area.

Trap 1/2
Return to the intersection and swim through the flooded tunnel. Open the door ahead and the trap is right in front of you by a valve.

Explorer 18/32
Go into the next room and pick up a valve from the shelf. Place it on the wall where the trap was. Turn the valve to temporarily open a gate in the water (you would've swam past it earlier). Swim through and you'll exit in the middle of a hallway. Follow the arrow sign left to a radio transmitter.

Achievement: To be or not to be
Go to the other end of the hallway and jump off the wooden balcony. Walk under and look right to see the skull displayed between two doors. Pick it up with telekinesis.

Note 12/15
Enter the door at the other end of the room opposite the skull. In this room is another device that requires a yellow tank. On a table nearby is the note.

Return to the skull and find a yellow tank in the room next to it. Use it on the device near Note #12 to unlock the door.

Explorer 19/32
Proceed past some dungeon cells until you reach more mines. Follow the right wall and when you pass some gas tanks, the Doctor will spawn again. Keep following the path. When you reach the intersection, take a left then a quick right. Going right at the next intersection leads to the duct to exit the area, but continue straight ahead. Keep going until you reach a lever (that doesn't work) and another intersection. Going left leads to a small room with food.

To exit, go right at the intersection earlier to reach a room with lots of yellow tanks. Take one back to the duct I mentioned earlier. When you reach the duct, either throw it over the fence or take the tank with you inside the duct.

Note 13/15
The note is in the duct you crawl into when escaping from the Doctor.

Trap 2/2
After exiting the duct, look left to see the trap right in front of you in the next room.

Note 14/15
In the same room as Trap #2, the note is inside the locker.

Note 15/15
Walk up the steps in the room to a "Laboratory Surface Access" button and "Manual Override" lever. The note is on top of the button.

To finish the level, use the yellow tank on the device in the room and switch it on. Turn the steam valve to half pressure. Switch on manual overrride then press the laboratory surface button.

The door leads back to trap door and Note #11. From there, go back up the stairs and open the exit door. Go down the hall to end the level.

Note 1/12
On the floor in front of you where you wake up.

Achievement: Firestarter, Burn baby! Burn!
The room you exit in is perfect for these achievements. Grab anything made of wood and use the torch in the cell you woke up in to ignite it.

Note 2/12; Explorer 20/32
Exit the cell and directly opposite is a small cubby with the note on a desk.

Note 3/12
On the other side of the room are two more cells. The note is inside the left one.

Explorer 21/32
Right of the two cells described above is another hall. Head down this hall to the Energy Supplement at the end find some food hidden in the room left of it. Explore this entire area to be sure it registers.

Note 4/12
Left of the two cells described above is another hallway. Go down this hallway to find the note in the first cell on your left.

Note 5/12
Continue down the hall and flip the lever at the end of the hall to open the gate back at the start of the hall. Enter the room beyond the gates to find the note pinned the wall near a lever.

Note 6/12; Explorer 22/32
Use the lever to unlock a door in the starting area, but unfortunately we can't return the way we came. Break the planks blocking the doorway (throw a barrel to break through the second layer) and crawl through the hole in the wall and fall down the long tunnel. Swim through the tunnel to a fork with some glowing mushrooms. Go right to find the note on a desk along with supplements.

Note 7/12; Explorer 23/32
Return to the previous fork and go left up some steps to another fork. Go left to find the note on a desk.

Note 8/12
Return to the fork and go right this time. Find the note by the campfire under a brick.

Note 9/12; Explorer 24/32
Climb up the ladder near the campfire and look right to see the note on a table.

Explorer 25/32
Burn the tree roots blocking your way then follow the left wall to an area overlooking a waterway. Go down the ladder by the burning corpse sack and explore a room in the lower area with the yellow tanks. Take a yellow tank with you back to the upper floor.

Trap 1/3
At the other end of the balcony, on your right is a small room with a trap.

Note 10/12; Explorer 26/32
Beyond the trap above is a hole in the wall. Crawl through to reach the radio transmitter and another note. You can repair the transmitter with the yellow tanks you found earlier.

Trap 2/3
Return to the fork just after burning the tree roots and go right this time to find a ladder. Climb up, then turn around and jump across the gap to find the trap on the floor in front of you.

Trap 3/3
In the next room directly after Trap #2, this trap is on the floor by the energy supplement.

Note 11/12
Break the planks blocking the doorway in the room with Trap #3 and proceed through to find the note on a desk.

Fall through the hole in the floor in the room with Note #11 to finally return to the hallway with Note #4. Go through the door you unlocked near the beginning of the level and find a lever behind a pillar which opens the nearby gate.

Open the next gate with telekinesis. Go down the hall and the Doctor will eventually spawn behind you. The hall is a dead end and the gate at the end doesn't open until the Doctor sees you. So backtrack through the hall until you see the Doctor to initiate a chase sequence. Use a lever to open a gate along the way, then turn right and run towards the daylight.

Note 12/12
After falling into the water, you'll come to more prison cells. Go left at fork further in to find the note in a cell at the end.

Return to the fork and open the steel doors straight ahead to finish the level.

There's no way to get lost at the start, so loop around the rooms until you see a lever you can pull from the window which opens the nearby door. Go inside and down the ladder. It breaks before you reach the bottom, preventing you from climbing back up.

Note 1/5
After falling down the broken ladder, turn left and into what looks like dining room. The note is nailed onto the table with a knife.

Trap 1/3; Explorer 27/32
Connected to the dining room is a side room filled with bones. On the floor near the box is the trap.

Note 2/5
Exit the dining area and walk down the hall towards the crying person. Use telekinesis to pull the lever to free her, and also opening the gate next to the dining area. Enter here to reach a room with guillotines and break the planks blocking the next doorway. Down this hall next to the corpse is the note.

Flip the next lever and you'll find the crying person gone. Flip the lever on one of the pillars in the area in which she was imprisoned to open two more doors. The crying woman (Mother) is the new enemy who will will spawn when you go down one of the two halls.

This area is basically a rectangle and you start in one corner. Facing the two open doors, there's a hidden lever (behind some crates) in a room at the end of the hallway on the left. Pulling the lever will open the exit door that's in the opposite corner from where you started (remember this area's a rectangle).

The Mother moves randomly, but she seems to stick to the 'rectangle'. Personally, I just took it slow and sneaked behind her from a distance and only moved when she was a fair distance ahead. If you're spotted you can outrun her pretty easily by circling the area to the opposite side.

Trap 2/3
After avoiding the Mother and escaping through the door, go through the cremation room and you'll reach a large cavern with a raised bridge. Instead of using the bridge, enter the other door. Follow the dark hallway as it turns right, and before the table with the energy supplement and lantern is the trap on the floor.

Note 3/5; Explorer 28/32
Left from the trap is a room with a radio transmitter. The note is on a shelf near the entrance.

Return to the bridge. Pull it down with telekinesis and cross to the other side. Climb up the crates and work your way along the wooden scaffolding. You will need to arrange some crates and barrels to make the jump to the higher levels. Your destination is a balcony high above where you entered the room.

Trap 3/3
On the higher balcony, go left. Before the door there is the trap on the floor.

Explorer 29/32
Return to where you climbed up and go right this time to reach a broken radio transmitter. Left of the transmitter is a room you can enter. Inside is a device that needs a yellow tank.

Note 4/5; Explorer 30/32
Go right from the broken transmitter, left down the stairs and right into the boarded up room that contains the note. The radio transmitter can be repaired with the yellow tank as well.

Whether you repair the radio transmitter or not, return to the intersection before Note #4. Go left from here to a ledge with a wooden cross. Jump down to the lower level from here. Enter the doorway with the flaming torch to reach the catacombs.

Explorer 31/32
Proceed through a flooded room and into the next one. There's a doorway boarded up across from where you enter. Enter that room and explore it for items.

Note 5/5
Return to the previous room and drop down to find the note on a shelf.

Enter the fancy door left of Note #5 to finish the level.

Achievement: Escape Artist
The achievement for disarming all traps unlocked for me at the start of the level despite there being one more trap in this level.

This level introduces a new mechanic: weighted switches. Explore the hallways you start in to find a rock and a floor plate. Place the rock on the floor plate to open the door then push it off again to open the door further in. Continue down the hall and jump to the lower level.

In front of you are two doorways. Go left and left again to find a single rock. Pick up the rock go forwards to a long hallway that stretches left and right. Go right, and a quick left into another short hallway. This hallway leads to another long hallway that runs parallel to the long one from before.

Place the rock somewhere in this second long hallway. Turn right and if you look into the room on your left, you'll see a hole in the wall. Ignore it for now and enter the room at the end of the hallway. In here are two more rocks. Pick up one of them and place it beyond the hole in the wall.

Go back and grab the other rock and take it to the long hallway with the first rock. Now travel down this long hallway with both rocks until you reach the room at the end. Look around the corner on the right here and you should see the room beyond the hole in the wall through the barred window. Place both rocks near the window.

Walking through the next doorway will spawn the Mother.

Explorer 32/32
Run down the hall into the cubby on the left. There are two Energy and one Health Supplements here.

Opposite the cubby is a barred window with a gap at the top. If you did like I said and placed both rocks near the barred window earlier, quickly grab both with telekinesis and send them through the gap.

Sprint back to the hole in the wall and crawl through. You are safe from the Mother now.

Place all three rocks on the floor switches and open the door. One switch is on your side, and the other two are on the other side.

Note 1/6
After the room with the three switches, search the next room to find the note nailed to a pillar.

Note 2/6
From the previous note, proceed down the hall and take the elevator up. In the next room is the note on the railing.

Note 3/6
Cross the bridge in the large, open cavern to find an altar in the centre. In the corner of the altar is the note.

Trap 1/1
Go right from the altar to find a series of hallways. Straight ahead as you enter is the final trap just before the door.

Note 4/6
Go down the long hall from the trap to find the note on the desk among a bunch of supplements.

Note 5/6
Further down the hall is a path on the left that leads to a ledge overlooking the cavern. The note is here next one half of the gate key.

Pick up the key and leave the room for a short scene. Return to the altar and jump across the broken bridge to the large gate.

Note 6/6
On the floor by the corpse is the last note.

Pick up the other half of the key and use it on the gate. Use your telekinesis on the gate one last time to finish the level and the game.