On March 24, 1984, five students report for Saturday morning detention at Shermer High School in the Chicago, Illinois suburb of Shermer. While not complete strangers, each of them comes from a different clique, and they seem to have nothing in common: the beauty and pampered Claire Standish (a "princess"), the sports jock Andrew Clark (an "athlete"), the bookish Brian Johnson (a "brain"), the reclusive outcast Allison Reynolds (a "basket case") and the rebellious John Bender (a "criminal").
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 stalking, frequently arguing, and at one point, 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 an abusive household, Brian has attempted 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; Allison's parents ignore her, Andrew's father forces him into sports, John's dad abuses him and his mother, Brian's parents put immense pressure on him to get good grades, and Claire's 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.
Despite the differences they all have in terms of their social statuses, the group begins to form friendships (and even relationships) as the day goes on. Claire makes it her mission to show Allison just how pretty she really is; Allison's new look sparks the romantic interest of Andrew who is taken aback by her new girly appearance. Claire decides to break her "pristine" virgin appearances by kissing Bender in the closet. Although it's well known by all of the them that the relationships would end with detention, they formed bonds they never would have expected that changed the way they would look at their peers forever.
Toward 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 "in the simplest terms, in the most convenient definitions": a brain, an athlete, a basket case, a princess, and a criminal.