The Breakfast Club is set in the fictional town of Shermer, Illinois. On Saturday, March 24, 1984, at 7:00 a.m., five students arrive at Shermer High School to check in for their eight-hour detention. On the face of it, the students have nothing in common aside from the punishment they have been given. Claire Standish is a popular girl, beautiful but snobbish and entitled. Andrew Clark is a jock, the standout on the wrestling team. Allison Reynolds, a gothic loner, and Brian Johnson, a high-achieving nerd, are not in the cool crowd and have spent their high school lives as outsiders. John Bender—the only student who isn't dropped off by a parent—is a rebellious troublemaker who likes to push people's buttons to provoke reactions.
The detention is overseen by Assistant Principal Richard Vernon. He assembles the students in the atrium-like open space of the school library and tells the kids not to move from their seats or talk, until they leave at 4:00 p.m. They are also not allowed to sleep. Vernon says they must write a thousand-word essay on the subject of "who you think you are." Vernon goes to sit at an office in an adjacent room, leaving the door open.
Bender, a regular in detention, ignores the rules, tormenting the other students and sexually harassing Claire, saying they should "knock up the prom queen." Claire argues with Bender, saying his resentment toward popular students has to do with his own fear of being rejected by others. When Vernon leaves his office briefly, Bender removes a screw from the adjoining room's door so it won't stay open, giving the students greater privacy.
While mindlessly tearing pages out of books to kill time, Bender enters into a discussion with Claire and Andrew about Claire's divorced parents, both of whom she resents for using her to get back at each other. Brian says he doesn't get along with his parents either. Bender tells Brian that a dork like him is a parent's "wet dream." When Bender returns to sexually harassing Claire, demanding to know whether she is a virgin, Andrew defends her by putting Bender in a wrestling hold and easily pinning him. When Bender gets up, he pulls out his knife and warns Andrew against further fighting.
While the kids eat lunch, Bender does a mocking impression of Brian's parents, portraying them as stereotypically supportive. He then shares an impression of his own father, implying that his father beats him. When Andrew calls Bender a liar, Bender shows a cigar burn on his forearm and says that's what you get at his house for spilling paint in the garage.
When Vernon leaves the office again, Bender leads the students out to the hallways so he can collect a bag of cannabis from his locker. They try to sneak back in but almost run into Vernon as he strolls the halls. When it seems they'll all get in trouble, Bender runs off on his own, diverting Vernon to the gym so the others can return to the library. Vernon punishes Bender by locking him in a storage room. He also threatens Bender with physical violence, asking Bender to throw the first punch so he can hit back.
Bender climbs through the duct system to escape the storage room. He falls through the ceiling in the library, then hides under Claire's desk when Vernon comes in to ask what that noise was. The students pretend not to know. After Vernon leaves them, Bender rolls joints that the students smoke. While high, the students go through each other's purses and wallets and ask questions. Andrew and Allison have a private moment together in which they bond over Allison's difficult home life.
Once they are all sitting together, the students talk about the craziest thing they'd do for a million dollars. Allison reveals that she is a nymphomaniac and has slept with her psychiatrist; then she demands to know whether Claire is a virgin. Claire, reluctant to tell the others, eventually admits she hasn't had sex. Allison then admits she is a compulsive liar and has never had sex.
Andrew reveals why he got detention, saying he taped a weaker teammate's buttocks together. He did it because he thought it was the sort of hi-jinx his father would be proud of, but afterward, Andrew considered the kid's humiliation and felt remorse. Andrew says that he hates his father, who is obsessed with winning and aggression. Andrew wishes he would get injured so he could have an excuse to stop wrestling.
As the other students bond over random talents they have, Bender erupts in anger at Claire. He says she got everything in life, insinuating that she has no right to complain because she is so privileged. He says that his Christmas gift was a carton of cigarettes, so Claire has no right to cry in front of them. Claire says she hates Bender.
Brian asks the others if they think they will still treat each other as friends on Monday. Claire says that it's her honest opinion that they won't. Brian tells her he would never ignore or humiliate any of them. He calls Claire conceited for caring so much about what her popular friends think. Claire says Brian doesn't know what the pressure is like. Brian says he does know about pressure, admitting that he is failing shop class because he failed to create a functional ceramic elephant lamp. Despairing over his grades, Brian brought a flare gun to school, thinking he might commit suicide with it. But the gun went off in his locker and he was given detention. He sobs as he speaks, but eventually laughs with the others over the absurdity of the flare gun going off. Allison admits that she is in detention because she didn't have anything better to do that day.
The students dance around the library, listening to music. Bender climbs through the duct system to sneak back into the closet he was locked in. Claire asks Brian to write one essay on all the others' behalf; he says he will. She then pulls Allison away to give her a makeover. When done, Claire finds Bender in the closet and kisses his collar. He says he would be a great way to get back at both her parents. Allison walks into the library transformed. No longer wearing black, she is in a pink dress. Andrew locks eyes and moves toward her, saying he likes that he can see her face now that her fringe isn't in her eyes.
The students walk down the hall as a group at the end of their detention. Andrew and Allison kiss outside before getting into their respective parents' cars. Claire removes an earring and puts it in Bender's hand before getting into her parent's car. Meanwhile, Vernon reads the words Brian left for him: "We found out that each of us is a brain, and an athlete, and a basket case, a princess, and a criminal. Does that answer your question? Sincerely yours, The Breakfast Club." The film ends with Bender putting the earring in his pierced lobe and then walking home across the football field. He raises his fist in the air triumphantly.
