As the film begins, Brandon and Phillip, roommates (and perhaps more), conspire to strangle a mutual acquaintance named David Kentley. The murder is committed with a simple piece of rope. They store the corpse in a simple luggage trunk used to hold rare books. Later that evening, they will take the body up to Connecticut and take a short vacation. But before they leave town, they will invite David's family, girlfriend, former best friend, and house master from prep school over to have a dinner party centered around the trunk.
The boys share a toast to committing the perfect murder. With all the evidence hidden away, including David's body, they prepare for the dinner. The housekeeper Mrs. Wilson arrives and begins helping them prepare, just in time for the guests to begin pouring in. The first to arrive is Kenneth, David's former best friend and the ex-boyfriend of David's current girlfriend. Then, Mr. Kentley, David's father, arrives along with David's aunt, a horoscope-reading British woman named Mrs. Atwater. When Mrs. Atwater first meets Kenneth, she confuses him with David, a misidentification which causes the nervous Phillip to break a glass in his hand. Then Janet, David's current girlfriend, arrives, and greets the other guests. The last to arrive is Rupert Cadell, the boys' former house master, whom Brandon and Phillip think of as having inspired the murder to begin with—he is the one who taught them about the Nietzschean concept of ubermenschen, or supermen.
After Cadell arrives, the guests sit down to eat. Phillip decides not to eat the chicken, much to everyone's surprise (vegetarianism does not fly with this set). Brandon explains that Phillip once had a job choking chickens and on one memorable occasion, a chicken that he had choked came back to life on the dinner table. Philip vehemently denies such a story, but Cadell seems confused, as he knows the story to be true.
Then, Cadell begins a discussion of what he sees as the "perfect murder," a lesson that clearly left a big impression on his impressionable young pupils. Murder, according to him, is an art, one which should remain in the exclusive domain of those superior enough to know how and when it should be utilized. Mr. Kentley, clearly offended, is moved to inquire how the superiority of those charged with such responsibility is to be decided. Kentley recognizes Brandon's response as sympathetic to the philosophy of Friedrich Nietzsche, whom, he points out, was also an influence on Hitler. Kentley is rattled by what he sees as an exceedingly irreverent and dangerous conversation and leaves the room.
Confidentially, Rupert asks Brandon if he is planning on committing a murder, but Brandon blithely deflects his inquiry. As the party continues, Mr. Kentley becomes more and more concerned about David’s unexplained tardiness. Meanwhile, in the absence of David, Brandon appears to be trying to manipulate Janet and Kenneth back into a romantic dyad. As the evening progresses, Phillip drinks heavily; he is clearly anxious. When Brandon goes so far as to tie up a bundle of books for Mr. Kentley with the rope that they used to kill David, Phillip becomes overwhelmed with nervousness. Rupert Cadell notices the unusual behavior of his two former students, and is compelled to question Phillip about David's potential whereabouts. Phillip tells him he doesn't know, but Rupert becomes more and more suspicious. Rupert suddenly begins a concerted effort to track down the whereabouts of the still-missing David. When Mrs. Kentley calls to report that David is not home, everybody starts to take their leave.
When Cadell goes to leave, Mrs. Wilson, the housekeeper, hands him the wrong hat, and as he takes it off, he notices the initials "D.K." inside, for David Kentley. He leaves, looking suspicious and anxious.
After everyone leaves, Brandon and Philip argue. Phillip has gotten exceedingly drunk, and feels certain that they will get caught. They are interrupted suddenly by the return of Cadell, who claims to have misplaced his cigarette case. We see Cadell place the cigarette case on the table out of sight of Brandon and Phillip, revealing to the audience that the missing case was just a ruse to get back in the apartment. After Brandon offers Cadell a drink, Cadell muses on the mystery of David Kentley's disappearance, even (on Brandon's prompting) going so far as to describe a hypothetical murder. Cadell is alarmed to notice that Brandon is holding a gun in his pocket, but when he points it out, Brandon laughs it off, suggesting that he is bringing it to Connecticut for protection. After Brandon puts the gun on the table, Cadell casually takes the murderous piece of rope out his pocket, suggesting to the boys that he knows that they killed David.
As Cadell absently plays with the rope, Phillip finally loses his last remaining grip and bursts into hysterical ramblings. Finally, Cadell discovers David's dead body in the trunk, and scolds the boys for twisting his philosophical investment in Nietzsche into an amoral excuse for the murder of an innocent peer. Cadell then walks to window, fires a gun into the air and turns to wait with Brandon and Phillip for the police to arrive.