The five strangers, 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 principal, Mr. Richard Vernon (Paul Gleason), and remain for a period of eight hours and fifty-four minutes (from 7:06 A.M. to 4 P.M.). He assigns a 1,000 word essay (in which each student must write about who they think 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 Andy and sexually harassing Claire. Allison remains oddly quiet except for the occasional random outburst.

The students pass the hours in a variety of ways: they dance, harass each other, tell stories, fight, smoke marijuana, and speak on a variety of subjects. Gradually they open up to each other and reveal their inner secrets (for example, Allison is a kleptomaniac and a compulsive liar and Brian is ashamed of his 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.