On Saturday, March 24, 1984, five students report at 7:00 a.m. for all-day detention at Shermer High School in Shermer, Illinois. While not complete strangers, each of them comes from a different clique, and they seem to have nothing in common: the beautiful and pampered Claire Standish, the state champion wrestler Andrew Clarke, the bookish Brian Johnson, the reclusive outcast Allison Reynolds, and the rebellious John Bender.
They gather in the high school library, where assistant principal Richard Vernon instructs them not to speak, move from their seats, or sleep for the next eight hours and 54 minutes. He assigns them a 1,000-word essay, in which each must describe "who you think you are." 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 talking, arguing, and, at one point, smoking cannabis 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 easily 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 due to their own problems, Andrew's father constantly criticizes his efforts at wrestling and pushes him as hard as possible, John's father verbally and physically abuses both John and his mother, Brian's overbearing parents put immense pressure on him to get good grades and keep it that way, and Claire's parents use her to get back at each other during frequent arguments. The students realize that even with their differences, they face similar pressures and complications in their lives.
Despite their differences in social status, the group begins to form friendships (and even romantic relationships) as the day progresses. Claire makes it her mission to show Allison just how pretty she really is, and Allison's new look sparks the romantic interest of Andrew, who is stunned when Allison's beauty is revealed. Claire decides to break her "pristine" virgin appearance by kissing Bender in the closet and giving him a [[hickey]]. Although they suspect that the relationships would end with the end of their detention, their mutual experiences would change the way they would look at their peers afterward.
As the detention comes to its final moments, the group requests that Brian complete the essay for everyone, and Brian agrees, leaving the essay in the library for Vernon to read after they leave. The students part ways outside the school. Allison and Andrew kiss, as do Claire and John. Allison rips Andrew's state champion patch from his etterman jacket to keep, and Claire gives John one of her diamond earrings, which he attaches to his earlobe. Vernon reads Brian's essay (read by Brian in voice-over), in which Brian states that Vernon 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." Brian signs the letter as "The Breakfast Club." As the movie ends, John raises his fist in triumph as he walks across the football field for home.