On March 24, 1984, five students—"criminal" John Bender, "athlete" Andrew Clark, "brain" Brian Johnson, "basket case" Allison Reynolds, and "princess" Claire Standish—report for Saturday morning detention at Shermer High School in the Chicago suburb of Shermer, Illinois. While not complete strangers, each of them comes from a different clique, and they seem to have nothing in common. They gather in the high school library, where they are instructed by the assistant principal, Richard Vernon, not to speak, move from their seats, or sleep for a period of eight hours, fifty-four minutes (from 7:06 a.m. to 4 p.m.). He assigns a 1,000-word essay to the students in which each must write about who he or she thinks he or she is. He then leaves, returning only occasionally to check on them. Bender, who has a particularly antagonistic relationship with Vernon, ignores the rules and frequently riles up the other students, teasing Brian and Andrew and harassing Claire. Allison is initially quiet except for the occasional random outburst.
The students pass the hours by arguing and smoking marijuana that Bender retrieves from his locker. Gradually, they open up to each other and reveal their deepest personal secrets: Allison is a compulsive liar, Andrew can't think for himself, John comes from a troubled household, Brian has contemplated suicide due to a bad grade, and Claire is a virgin who feels constant pressure from her friends. They also discover that they all have strained relationships with their parents; Alison's parents ignore her, Andrew's father forces him into sports, John's dad abuses him, Brian's parents put immense pressure on him to get good grades, and Claire's divorced parents use her to get back at each other. They fear making the same mistakes as the adults around them. The students realize that despite their differences, they face similar pressures and complications in their lives.
Towards the end of the day, the other students ask Brian to write the essay that Vernon assigned earlier. Brian does so, but instead of writing about the assigned topic, he writes a letter objecting to Vernon's request to describe who they are, stating that the man has already judged who they are (an athlete, a basket case, a princess, a brain, and a criminal), and that he will not accept any different accounts from them. Brian finishes the essay and leaves it at the table for Vernon to read after they leave. Before the students part ways outside the school, Allison and Andrew kiss, as do Claire and John; Claire gives John one of her diamond earrings as a parting gift. Brian then finishes the narration by signing it as "The Breakfast Club" as the movie ends with John raising his fist up in triumph as he leaves for home. Despite the formed friendships and relationships, whether or not the students returned to their different cliques and never spoke again or remained friends, is unclear.