As the detention nears its end, the group requests that Brian complete the essay for everyone, and John returns to the storage closet to fool Vernon into thinking he has not left. Brian leaves the essay in the library for Vernon to read after they leave. As the students part ways, Allison and Andrew kiss, as do Claire and John. Allison rips Andrew's state champion patch from his jacket to keep, and Claire gives John one of her diamond earrings, which he puts on. Vernon reads the essay, in which Brian states that Vernon has already judged who they are using stereotypes; in fact, the students found that "each one of us is a brain, an athlete, a basket case, a princess, and a criminal." He signs off the letter with "Sincerely yours, the Breakfast Club."

Claire gives Allison a makeover, which sparks romantic interest from Andrew. Claire decides to break her "pristine" virginal appearance by kissing John and giving him a hickey. Although they suspect their new relationships will end along with their detention, they believe their mutual experiences will change the way they look at their peers.

The students pass the hours by talking, arguing, and, at one point, smoking marijuana. Gradually, they open up and reveal their secrets: Claire has experiences of peer pressure, John comes from an abusive household, Allison is a compulsive liar, Andrew can't think for himself, and Brian contemplated suicide over a bad grade. They discover they all have poor relationships with their parents: Claire's parents use her to get back at each other during arguments, John's parents physically and verbally abuse him, Allison's parents ignore her, Andrew's father pushes him to the limit, especially in wrestling, and Brian's parents pressure him to earn high grades. The students realize that, despite their differences, they face similar problems.

On a Saturday, March 24, 1984, five high school students report at 7:00 am for all-day detention. Each comes from a different clique: stuck-up Claire Standish, geek Brian Johnson, wrestler Andrew Clark, rebellious John Bender, and outcast Allison Reynolds. They gather in the school library, where assistant principal Richard Vernon instructs them not to talk, move from the seats, or sleep until they are released at 4:00 p.m. He assigns them a thousand-word essay, in which each must describe "who you think you are". He leaves, returning only occasionally to check on them.

John, who has an antagonistic relationship with Vernon, ignores the rules and riles up the other students, teasing and harassing Brian, Andrew, and Claire. Vernon gives John eight weekends' worth of additional detention and eventually locks him in a storage closet, but he escapes and returns to the library.