The narrator is Charles Everett "Charlie" Decker, a senior at Placerville High School in fictional Placerville, Maine. The story begins in the morning, when Charlie is called to the principal's office from his Algebra 2 class. His principal, Mr. Denver, wants to speak to Charlie about an incident that occurred two months earlier, where Charlie had struck his chemistry teacher, Mr. Carlson, in the head with a heavy wrench. For unknown reasons, Charlie snaps and responds with a series of insulting remarks towards Mr. Denver, which prompts him to expel Charlie from Placerville High School. Charlie storms out of the principal's office and retrieves a semiautomatic pistol from his locker. After setting the contents of the locker on fire, he returns to his classroom and fatally shoots his teacher Mrs. Jean Underwood. The fire sets off an alarm and students are beginning to be evacuated. Charlie then shoots another teacher, Mr. Vance, after Vance enters the classroom to notify students of the alarm. The students and teachers evacuate the school and police and media arrive on the scene.

A long discussion ensues with his fellow students whom he holds hostage. Charlie claims that he honestly does not know why he did this, and claims that if he did know, he probably wouldn't do it to them. He also successfully attempts to win the approval of his fellow students. In the following few hours, Charlie toys with various authority figures who attempt to negotiate with him, including the principal, the school psychologist, and the chief of police. He also unwittingly turns the class into a sort of psychotherapy group, causing his schoolmates to semi-voluntarily tell embarrassing secrets about themselves and each other. Interspersed throughout are narrative flashbacks to Charlie's own troubled childhood, particularly his horrible relationship with his father. Other notable incidents include a violent disagreement between two female students, and a SWAT team sniper shooting Charlie in the chest (his padlock, which he placed in his breast pocket on a whim, stops the bullet).

He finally comes to the realization that one student is really being held there against his will: a seeming "big man on campus" named Ted Jones, who is harboring his own unpleasant secrets. Jones realizes this as well and attempts to flee, but is captured and brutally assaulted by the other students, leaving him badly battered and in a catatonic state. At 1:00 PM, Charlie releases the students. When police chief Frank Philbrick enters the classroom, the now-unarmed Charlie deliberately makes a "threatening" gesture and is shot three times. He survives being shot and is found not guilty by reason of insanity, and is committed to a psychiatric asylum in Augusta until he is no longer a threat to society. The story ends with Charlie addressing the reader that he has to "turn out his light".