The plot follows five students at fictional Shermer High School in the widely used John Hughes setting of Shermer, Illinois (a fictitious suburb of Chicago based on Hughes' hometown of Northbrook, Illinois), as they report for Saturday detention on March 24, 1984. While not complete strangers, the five are all from different cliques or social groups: John Bender (Judd Nelson) "The Criminal"; Claire Standish (Molly Ringwald) "The Princess"; Brian Johnson (Anthony Michael Hall) "The Brain"; Andy Clark (Emilio Estévez) "The Athlete"; and Allison Reynolds (Ally Sheedy) "The Basket Case". The school's disciplinary principal, Richard Vernon (Paul Gleason), gives them all an assignment; they will write an essay about "who you think you are" and the violations they committed to end up into Saturday detention.The seeming delinquent of the group, Bender, is instantly hostile toward his classmates, acting out several times. He acts as though he will urinate on the floor, suggests that he and Andrew close the library door and have forced sex with Claire, challenges Andrew's athletic prowess and rigs the main door to the library so that it can't be braced open so Vernon can keep an eye on them from his office. During lunch, he jovially mocks Brian's home life and then offers everyone a glimpse into his relationship with his own father who abuses him both verbally and physically. In a rage, Bender runs off and sits alone, hurt by what he revealed to the group.They pass the hours in a variety of ways: they dance, harass each other, tell stories, fight, smoke marijuana, and talk about a variety of subjects. Gradually they open up to each other and reveal their secrets, for example, Allison is a compulsive liar, and Brian and Claire are ashamed of their virginity and Andy got in trouble because of his overbearing father. They also discover that they all have strained relationships with their parents and are afraid of making the same mistakes as the adults around them. However, despite these evolving friendships, they are afraid that once the detention is over, they will return to their respective cliques and never speak to each other again.Mr. Vernon actually has several epiphanies of his own. When he's down in the basement looking through the personal files of his teachers, he's caught by the school's janitor, Carl Reed (John Kapelos), who essentially blackmails him for his silence about Vernon's poking through private information about his staff. The two spend the rest of the day talking. Vernon admits he's frightened of the future; that the very students he has in detention will one day be running the country. He also claims that since he's been into education for many years that the kids haven't changed, they are still defiant, arrogant and disrespectful of authority. Carl tells Vernon he's dead wrong, that Vernon is the one who's attitude has soured his perspective for a job he once liked. The kids will always be the way Vernon described them.The group decides to sneak out of the library and go to Bender's locker. Bender retrieves a small amount of marijuana hidden there. On their way back, they nearly run right into Vernon. While trying to find a route back to the library undiscovered, Bender sacrifices his own freedom to help the others escape. (He also stuffs his stash down Brian's pants.) Vernon catches up with him in the gym, shooting hoops. Vernon takes Bender to a small closet. With his obvious hatred for the student apparent, Vernon challenges him, offering him one defenseless punch. Bender is too scared to take the challenge and Vernon reminds him that people won't take the word of a delinquent student over that of a high school principal. He leaves Bender locked in the closet. Bender slips out through the ceiling and rejoins the group, retrieving his stash from Brian. The group spends the rest of their time smoking weed and even relaxing while Vernon talks to Carl in the basement. They loosen up, play music and dance. Bender sneaks back to his closet when the end of their detention approaches.Late in the day, some of the group talk about what they did to land into Saturday detention: Claire had skipped class to go out with friends. In an earlier moment, she worries about whether or not her parents will ground her and suggests that they have a strained marriage where they use Claire to get back at each other. Brian tells everyone that he had felt suicidal after failing a project in his shop class and had brought a flare gun to school to possibly kill himself (the gun had gone off in his locker, starting a fire). Andrew's story seems to be the most painful: he had attacked another student (a friend of Brian's) in the locker room after gym class, beating on him while his friends cheered him on and covering the boy's buttocks in duct tape, causing the boy minor but humiliating injury. Andrew says he did it because his father is an overbearing tyrant (he describes him as a "mindless machine I can't relate to anymore") who can't abide his kids being seen as losers and that they must win at all costs. Alison isn't clear about the reason why she's in the detention session other than to say she had had nothing better to do on a Saturday.In the end, some of their more hidden character traits emerge: Claire is a natural leader. Bender develops a softer attitude and becomes more friendly with everyone. Claire spends some time with him in the locked closet making out with him and it seems the two will try a romantic relationship. Andrew becomes interested in Allison after she allows Claire to give her a makeover. Brian realizes he can write very eloquently, as he gets to show everyone later.At Claire's request and the consensus of the group, Brian agrees to write the essay Mr. Vernon assigned earlier on behalf of them all, which challenges Vernon and his preconceived judgments about them. While Brian accedes, instead of writing about the actual topic, he writes a very motivating letter that is in essence, the main point of the story. He signs the essay "The Breakfast Club", and leaves it on the table for Vernon to read when they leave. There are two versions of this letter, one read at the beginning and one read at the end, and they differ slightly; illustrating the shift in the students' judgments of one another, and their realization that they truly have things in common. The beginning of the letter is as follows:"Saturday, March 24, 1984. Shermer High School, Shermer, Illinois 60062. Dear Mr. Vernon, we accept the fact that we had to sacrifice a whole Saturday in detention for whatever it was that we did wrong. What we did was wrong. But we think you're crazy to make us write this essay telling you who we think we are. What do you care? You see us as you want to see us... in the simplest terms and the most convenient definitions. You see us as a brain, an athlete, a basket case, a princess, and a criminal. Correct? That's how we saw each other at seven o'clock this morning. We were brainwashed."The letter read before the closing credits reads as follows:"Dear Mr. Vernon, we accept the fact that we had to sacrifice a whole Saturday in detention for whatever it was we did wrong, but we think you're crazy to make us write an essay telling you who we think we are. You see us as you want to see us... In the simplest terms and the most convenient definitions. But what we found out is that each one 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 letter is the focal point of the movie, as it demonstrates and illustrates the changes the students undergo during the course of the day; their attitudes and perspectives have changed and are now completely different. The movie ends as the characters leave detention. The final shot shows Bender walking near the goal post of the football field, freezing as he raises his hand triumphantly and fading to a dark frame as the ending credits roll.