The plot follows five students at fictional Shermer High School in the fictitious Chicago suburb of Shermer, Illinois as they report for Saturday detention on March 24, 1984. While not complete strangers, the five
teenagers are each from a different clique or social group.
The five students – Allison Reynolds (Ally Sheedy, the loner), Andrew Clark (Emilio Estevez, the jock), John Bender (Judd Nelson, the juvenile delinquent), Brian Johnson (Anthony Michael Hall, the nerd), and Claire Standish (Molly Ringwald, the preppie) – who seem to have nothing in common at first, come together at the high school library, where they are harangued and ordered not to speak or move from their seats by the antagonistic vice-principal, Richard Vernon (Paul Gleason). They are to remain for a period of eight hours, fifty-four minutes (from 7:06 A.M. to 4 P.M., the only indication of time being on a clock that is 20 minutes fast). He assigns a 1, 000 word essay (in which each student must write about who he or she thinks they are) and then leaves them mostly unsupervised, returning only occasionally to check on them. Bender, who has a particularly negative relationship with Mr. Vernon, disregards the rules and riles the other students; mocking Brian and Andrew, and sexually harassing Claire. Allison remains oddly quiet except for the occasional random outburst.
The students pass the hours in a variety of ways. Gradually they open up to each other and reveal their inner secrets (for example, Allison is a compulsive liar, Bender comes from an abusive household and Brian and Claire are ashamed of their virginity). 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 developing friendships the students are afraid that once the detention is over, they will return to their very different cliques and never speak to each other again.
At the request and consensus of the students, Brian is asked to write the essay Mr. Vernon assigned earlier (the subject of which was to be a synopsis by each student detailing “who you think you are” (sic)), which challenges Mr. Vernon and his preconceived judgments about all of them. Brian does so, but 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 as “The Breakfast Club” and leaves it at the table for Mr. Vernon to read when they leave. There are two versions of this letter, one read at the beginning and one at the end, which are slightly different; illustrating the change in the students’ judgments of one another and their realization that they truly have things in common.