The group's "delinquent," John Bender, is quite hostile towards his classmates and constantly acts out during detention. He acts as though he will urinate on the floor, suggests that he and Andy close the library door and have forced sex with Claire, challenges Andy's athletic abilities, and takes a screw out of the main door to the library so that Vernon can't keep an eye on them from his office.

While the group is eating lunch, he mocks Brian's home life. However, he eventually opens up about his relationship with his father, who abuses him. In a rage, John storms off and sits alone, hurt by what he revealed to the group.

While eating lunch, Vernon spills coffee everywhere and leaves his office for the faculty lounge. The group takes that opportunity to go to John's locker for weed. On their way back to the library, they run into Vernon and run back to the library, taking a different path to avoid him. After getting caught at a dead end, John stuffs the marijuana in Brian's pants and sacrifices himself so the group can return without Vernon catching them. Vernon then catches John in the gym dunking a basketball. Vernon brings him to a supply closet. Vernon makes his hatred for John very apparent and tells John to throw a punch at him. John sits there in shock and is too scared to take up Principal Vernon's challenge. He reminds John that no one will take the word of a delinquent like John over a respected high school principal. Vernon then leaves John in the locked supply closet all by himself. Moments later, John sneaks through the ceiling and rejoins the rest of the group in the library.

To pass the time, they dance, tell stories, argue, smoke marijuana, and harass each other. They gradually open up to each other and reveal their secrets. They also discover that they all have strained relationships with their parents. Despite their budding friendship, they are afraid that once detention is over, they will return to their respective social groups and never speak to each other again.

Principal Vernon heads to the basement and looks through his staff's files; however, the school's janitor, Carl, catches him in the act. He blackmails Vernon for his silence about Vernon digging through private information about his staff. The two spend the rest of the day talking in the basement. He claims that when they get older, the kids he's been teaching will eventually run the country. He also claims that in the 22 years he has been in education, the kids haven't changed; they are defiant, arrogant, and disrespectful. Carl tells Vernon that he is wrong and that the job he once liked soured his attitude.

The group later talks about what they did that landed them in Saturday detention. Andrew attacked another student, Brian's friend, in the locker room. He beat the student up and taped his butt cheeks together while his friends cheered him on. Andy reveals he did it because his father is an overbearing tyrant who can't tolerate his kids being seen as losers and that they must win at all costs. Brian reveals that he was feeling suicidal after failing a project in his shop class and had brought a flare gun to school to kill himself. The flare gun went off in his locker and started a fire. Allison had not done anything to get into detention. She claims she "had nothing better to do." Claire ended up in detention because she skipped class.

John sneaks back into the supply closet where Vernon left him. Upon Claire's request, Brian agrees to write the essay assigned by Vernon on behalf of the entire group. Claire gives Allison a makeover to make her look more girly, saying she would be prettier without the black stuff around her eyes. John develops a softer attitude and becomes friendlier with the group. Claire goes to the supply closet, where John is alone, and they kiss. After seeing Allison all dolled up, Andy gains interest in her, and Brian's "essay" challenges the assignment given to the group. The group leaves detention, and Allison and Andy share a kiss before going into their respective cars. Instead of answering the prompt, Brian's essay reads as follows:

"Dear Mr. Vernon, we accept the fact that we had to sacrifice a whole Saturday in detention for whatever it is we did 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. But what we found out is that each one of us is a brain, and an athlete, and a basketcase, a princess, and a criminal. Does that answer your question?

Sincerely yours, the Breakfast Club."

The letter is the movie's focal point, as it demonstrates and illustrates the changes the students undergo during the day; their attitudes and perspectives have changed and are now entirely different.

The final shot shows John walking near the football field's goalpost, freezing as he raises his hand triumphantly. The shot fades to a dark frame as the ending credits roll.

• Struggle with Identity: Brian is the academic overachiever and his self-worth is tied to his grades, and he fears the consequences of failure. His identity is wrapped up in being "the brainiac," which leaves little room for personal growth or emotional expression.

• Role Confusion: Brian is unsure who he is beyond his academic accomplishments. When he reveals his struggles with depression and the pressure to perform, it becomes clear that he feels trapped by the expectations others have for him.

• Struggle with Identity: Claire represents the stereotypical "popular girl," someone who is concerned with appearance, social status, and fitting in. She feels pressure to maintain her image, which prevents her from forming genuine connections.

• Role Confusion: Claire's interactions with the group reveal that she feels misunderstood and boxed in by her role as "the Princess." She is expected to act a certain way to fit into her social group, but she longs to break free from these expectations.

• Struggle with Identity: Andrew is the high school wrestler, living under the weight of his father's expectations to be the best. He struggles with the pressure to succeed in sports and the need to conform to the image of a tough, competitive athlete.

• Role Confusion: Andrew’s conflict arises from feeling trapped in the role of "the Jock." His confession about bullying another student to impress his father reveals that he is unsure of who he is outside of his athletic persona.

• Struggle with Identity: Allison is the introverted, socially awkward outsider who feels invisible and disconnected from her peers. She struggles with a sense of abandonment and self-worth.

• Role Confusion: Allison is unsure of her place in the social world and hides behind her eccentric behavior to avoid being hurt. She uses her "weirdness" as a shield, but it also isolates her from meaningful interactions.

• Struggle with Identity: Bender comes from a troubled home and plays the role of the rebellious "bad boy" as a way to cope with his anger and pain. He often acts out, using sarcasm and aggression to hide his vulnerability.

• Role Confusion: Bender’s struggle is one of deep insecurity, unsure of who he is beyond the role of "the Criminal." His tough exterior hides a desire for acceptance and love, but he’s afraid of being vulnerable.

• Struggle with Generativity:

  Rather than fostering an environment of growth, Vernon tries to impose authority by controlling and belittling the students. His actions reveal that he feels more comfortable exerting power than understanding or guiding them, possibly because he feels disconnected from the students’ generation and struggles to empathize with them.

• Stagnation: Throughout the film, Vernon expresses disdain for the students, referring to them with disdainful comments like "You’re all going to be back here on Monday.” This reflects a lack of faith in their potential and suggests a feeling of stagnation in his own life, leading him to project negativity onto the students. In addition, Vernon’s interactions with the janitor, Carl, provide a brief glimpse into his dissatisfaction with his job. In a moment of vulnerability, he admits he feels misunderstood and disconnected from others, which implies he may feel isolated and unsure about his contribution to the school and the community.