On Saturday, March 24, 1984, five students at the fictional Shermer High School in Shermer, Illinois, report for an all-day detention: socially awkward Brian Johnson, volatile wrestler Andrew Clark, shy loner Allison Reynolds, popular girl Claire Standish, and rebellious delinquent John Bender. They gather in the school library and meet with their vice principal Richard Vernon, who warns them not to talk or move from their seats and assigns each of them the task of writing a thousand-word essay describing "who you think you are."
Bender ignores the rules and spends his time antagonizing the others and defying Vernon, who gives him eight additional weekends of detention. The students sneak off to retrieve Bender's marijuana stash from his locker. Still, when they see Vernon returning to the library, Bender deliberately gets caught to allow the others to sneak back in. Locked in a storage closet as punishment, Bender is berated by Vernon even further who tells him that he wants Bender to prove how tough he is and is offered a chance to punch Vernon but fails to act. He soon escapes into the ceiling panels and falls into the library, where the others hide him from Vernon.
The students pass the time arguing, listening to music, and smoking marijuana, gradually opening up about their home lives and their reasons for being in detention:
- Claire's popularity subjects her to intense peer pressure, while her bickering parents use her against each other; she received detention for skipping school to go shopping.
- Bender reveals the physical abuse he and his mother suffer at the hands of his father, including cigar burns, as well as other injuries he has sustained from his father physically abusing him. He is serving detention for pulling a false fire alarm.
- Andrew became influenced by jock culture and is intimidated by his father to succeed in wrestling. He was given detention for taping a student's buttocks together in an attempt to win both his teammates' and his father's approval.
- Brian is under so much academic pressure from his parents to get good grades that he contemplated suicide after getting an F in shop class. He was sent to detention for bringing a flare gun to school to kill himself.
- Allison is compulsively dishonest with neglectful parents, constantly stealing things to use should she ever run away from home. She admits that she showed up to detention for lack of anything better to do.
Despite their differences, the students realize they all face similar problems; Andrew and Allison bond over their complex relationships with their parents; Brian and Claire each feel anxiety over being a virgin. Still, the group suspects that their new friendships will end once detention is over. Meanwhile, Vernon complains to the janitor, Carl, that students have become less disciplined, but Carl suggests that Vernon is the one who has changed and cares too much about what his students think of him.
The others convince Brian to complete Vernon's essay on the group's behalf. Claire gives Allison a makeover, which sparks romantic interest from Andrew, and Bender crawls back to the closet, where Claire challenges her "pristine" reputation by kissing him. As the students part ways, Allison and Andrew kiss, and she rips the state championship patch off his letterman jacket to keep; Claire gives Bender one of her diamond earrings, and they share their own kiss.
Vernon reads Brian's essay, which asserts that the vice principal has made petty assumptions about all of them and declares that "each one of us is a brain (Brian), an athlete (Andrew), a basket case (Allison), a princess (Claire), and a criminal (Bender)." "Does that answer your question? Sincerely yours, the Breakfast Club." The film then shifts to Bender, who triumphantly raises his fist in the air while walking home across the school's football field.
