Garcin: Hell is -- other people.Three humans die and find themselves in a room, slowly realizing that they are each other's tormentors. They discuss their situation, try to resist it, foil each other's efforts, and finally acquiesce.
At some point, Garcin frets over whether he had acted cowardly before he died. Specifically, he questions whether standing by one's ideals is necessarily decent, and whether it is not possible that alongside his idealistic motives, he was also motivated by shameful fear. "Is it right to judge a life by a single action?", he pleads.
All three main characters are villainous, but Garcin is the one who receives our empathy, because he is concerned with redemption, and with introspection aimed at self-salvation. Estelle is only bent on finding solace and forgetfulness in being seen and desired by a man, whereas Inez is wholly unrepentant, and simply seeks to continue her parasitic existence by preying on Garcin.
All three are locked in a vicious circle of oppressive need -- Garcin needs validation from Inez, who needs Estelle's validation, who in turn needs Garcin to feel validated. And so Sartre demonstrates that L'enfer is, indeed, les autres.