The plot follows five students at fictional Shermer High School in the widely used John Hughes setting of Shermer, Illinois (a fictitious suburb of Chicago based on Hughes' hometown of Northbrook, Illinois), as they report for Saturday detention on March 24, 1984.
While not complete strangers, the five are all from different cliques or social groups: John Bender (Judd Nelson) "The Criminal"; Claire Standish (Molly Ringwald) "The Princess"; Brian Johnson (Anthony Michael Hall) "The Brain"; Andy Clark (Emilio Estévez) "The Athlete"; and Allison Reynolds (Ally Sheedy) "The Empty Basket Case". The school's disciplinary principal, Mr. Vernon, gives them all an assignment; they will write an essay about "who you think you are" and the violations they committed to end up in Saturday detention.
They pass the hours in a variety of ways: they dance, harass each other, tell stories, fight, smoke marijuana, and talk about a variety of subjects. Gradually they open up to each other and reveal their secrets, for example, Allison is a compulsive liar, and Brian and Claire are ashamed of their virginity and Andy got in trouble because of his overbearing father. 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 evolving friendships, they're afraid that once the detention is over, they will return to their respective cliques and never speak to each other again.
In the end, some of their more hidden character traits emerge: Claire emerges as a natural leader. Bender develops a softer attitude and becomes more friendly with everyone. Claire even kisses him and it seems the two will try a romantic relationship. Andrew becomes interested in Allison after she allows Claire to give her a makeover.
At Claire's request and the consensus of the group, Brian agrees to write the essay Mr. Vernon assigned earlier, which challenges Mr. Vernon and his preconceived judgments about all of them. While Brian accedes, 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 "The Breakfast Club", and leaves it on 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, and they differ slightly; illustrating the shift in the students' judgments of one another, and their realization that they truly have things in common. The beginning of the letter is as follows:
"Saturday, March 24, 1984 Brian Johnson [although unknown at this point] Shermer High School Shermer, Illinois 60062
Dear Mr. Vernon:
We accept the fact that we had to sacrifice a whole Saturday in detention for whatever it was that we did wrong. What we did was wrong. But we think you're crazy to make us write this essay telling you who we think we are. What do you care? You see us as you want to see us... in the simplest terms and the most convenient definitions. You see us as a brain, an athlete, a basket case, a princess, and a criminal. Correct? That's how we saw each other at seven o'clock this morning. We were brainwashed.
The letter read before the closing credits reads as follows:
"Dear Mr. Vernon:
We accept the fact that we had to sacrifice a whole Saturday in detention for whatever it was we did wrong, but we think you're crazy to make us write an essay telling you who we think we are. You see us as you want to see us... In the simplest terms and the most convenient definitions. But what we found out is that each one of us is a brain... ...and an athlete... ...and a basket case... ...a princess... ...and a criminal.
Does that answer your question? Sincerely yours, the Breakfast Club.
The letter is the focal point of the film, as it demonstrates and illustrates the changes the students undergo during the course of the day; their attitudes and perspectives have changed and are now completely different. The movie ends as the characters leave detention. The final shot shows Bender walking near the goal post of the football field, freezing as he raises his hand triumphantly and fading to a dark frame as the credits roll.