Three students of an Austrian boarding school, Reiting, Beineberg and Törless, catch their classmate Basini stealing money from one of the three and decide to punish him themselves instead of turning him in to the school authorities. They start with bullying and abusing him, first physically and then psychologically and sexually, while also blackmailing him by threatening to denounce him. Their abusive treatment of Basini becomes openly sexual and increasingly sadistic; nevertheless, he endures all the torture even when, after being deprived of any dignity, he is discredited by the entire class.
Törless' moral and sexual confusion leads him to join Beineberg and Reiting's degradation of Basini; he is both sexually attracted to Basini and Beineberg and repelled by them. He observes and takes part in the torture and rape of Basini while telling himself that he is trying to understand the gap between his rational self and his obscure irrational self; he is a disturbed and despairing observer of his own states of consciousness. Basini professes love for Törless and Törless comes to reciprocate somewhat, but is ultimately repelled by Basini's unwillingness to stand up for himself. This disgust with Basini's passivity ultimately leads him almost off-handedly to stand up to Beineberg and Reiting. When the torment becomes unbearable, Törless covertly advises Basini to turn himself in to the headmaster as a way out of the situation.
An investigation is commenced, but the only party to be found guilty is Basini. Törless makes a strange existential speech to the school authorities about the gap between the rational and irrational ("...after all, things just happen"), which puzzles the authorities more than anything else. They decide he is of too refined an intellect for the institute, and suggest to his parents that he be privately educated, a conclusion that he comes to on his own.
Other subplots include Törless' experience with the local prostitute Božena, his encounter with his mathematics teacher, and his analysis of his parents' attitudes toward the world.
