Three damned souls, Joseph Garcin, Inèz 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 French 'Second Empire'. At first, none of them will admit the reason for their damnation: Garcin says that he was executed for being an outspoken pacifist, while Estelle insists that a mistake has been made; Inèz, 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.
Garcin suggests that they try to leave each other alone and to be silent, but Inèz starts to sing about execution and Estelle vainly wants to find a mirror to check on her appearance. Inèz 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 Inèz is attracted to Estelle, Estelle is attracted to Garcin, and Garcin is not attracted to either of the two women.
After arguing, they decide to confess to their crimes so they know what to expect from each other. Garcin cheated on and mistreated his wife, and was executed by firing squad for desertion; Inèz is a manipulative sadist who seduced her cousin's wife, Florence, while living with them—which drove the cousin to kill himself, and resulted in Florence asphyxiating herself and Inèz by flooding the room with gas while they slept, out of guilt—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. Garcin finally begins giving in to the lascivious Estelle's escalating attempts to seduce him, which drives Inèz crazy. Garcin is constantly interrupted by his own guilt, however, and begs Estelle to tell him he is not a coward for attempting to flee his country during wartime. While she complies, Inèz mockingly tells him that Estelle is just feigning attraction to him so that she can be with a man—any man.
This causes Garcin 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 Inèz that he is not cowardly. She refuses, saying that he is obviously a coward, and promising to make him miserable forever. Garcin concludes that rather than torture devices or physical punishment, "hell is other people." Estelle tries to persevere in her seduction of Garcin, but he says that he cannot make love while Inèz is watching. Estelle, infuriated, picks up a paper knife and repeatedly stabs Inèz. Inèz chides Estelle, saying that they are all already dead, and even furiously stabs herself to prove that point. As Estelle begins to laugh hysterically at the idea of them being dead and trapped together forever, the others join in a prolonged fit of laughter before Garcin finally concludes, "Eh bien, continuons..." ("Well then, let's get on with it...").