Three damned souls, Joseph Garcin, Inès Serrano, and Estelle Rigault, are brought to the same room in Hell and locked inside by a mysterious valet. They had all expected torture devices to punish them for eternity, but instead find a plain room furnished in the style of the Second French Empire. At first, none of them will admit the reason for their damnation: Joseph says that he was executed for being a pacifist, while Estelle insists that a mistake has been made; Inès, however, is the only one to demand that they all stop lying to themselves and confess to their moral crimes. She refuses to believe that they have all ended up in the room by accident and soon realizes that they have been placed together to make each other miserable; she deduces that they are to be one another's torturers. Joseph suggests that they try to leave each other alone and to be silent, but Inès starts to sing about an execution and Estelle vainly wants to find a mirror to check on her appearance. Inès tries to seduce Estelle by offering to be her "mirror" by telling her everything she sees, but ends up frightening her instead. It is soon clear that Inès is attracted to Estelle, Estelle is attracted to Joseph, and Joseph is in no mood for sexual behavior.
After arguing, they decide to confess to their crimes so they know what to expect from each other. Joseph cheated on and mistreated his wife; Inès seduced her cousin's wife while living with them; and Estelle had an affair and then killed the resulting child, prompting the child's father to commit suicide. Despite their revelations, they continue to get on each other's nerves. Joseph finally begins giving in to the lascivious Estelle's escalating attempts to seduce him, which drives Inès crazy. Joseph is constantly interrupted by his own thought, though, and begs Estelle to tell him he is not a coward for attempting to flee his country during wartime. While she complies, Inès tells him that Estelle is just feigning attraction to him so that she can be with a man – any man. This causes Joseph to abruptly attempt an escape. After his trying to open the door repeatedly, it inexplicably and suddenly opens, but he is unable to bring himself to leave, and the others remain as well. He says that he will not be saved until he can convince Inès to trust in him. She refuses, saying that he is obviously a coward, and promising to make him miserable forever. Joseph concludes that rather than torture devices or physical punishment, "hell is other people." Estelle tries to persevere in her seduction of Joseph, but he says that he cannot make love while Inès is watching. Estelle, infuriated, picks up a paper knife and repeatedly stabs Inès. As they are all already dead, this attack does nothing and Inès even halfheartedly stabs herself, beginning to laugh. As Estelle comments on the idea of their being trapped here forever and laughs too, all three join in prolonged fit of laughter before Joseph finally concludes, "Eh bien, continuons" (roughly "Eh well, let's continue...").