In Pennsylvania, a black mining engineer Ralph Burton, is trapped in a cave-in for five days when the tunnel he is inspecting collapses. When sounds of rescue work cease, Ralph, in a rage that he is being left to die, digs until he reaches a ladder and climbs out. He finds the mine deserted, then sees newspaper headlines reading, "U.N. Retaliates for Use of Atomic Poison" and "Millions Flee from Cities! End of the World." In the deserted town, Ralph hotwires a car, then drives to New York City, but finds the George Washington Bridge and Lincoln Tunnel clogged with empty cars. At a shipyard, Ralph finds a motorboat, which he navigates to the city docks. Ralph's shouts through empty streets bring only echoes, and at a vacant church, he cries in anguish. At a radio transmitting station, he listens to a recording of a broadcast in which he learns that a war started when a rogue nation began using radioactive isotopes to poison the world. Ralph fears that he may be the only person left alive in the world, but after he surveys his domain from the top of the Empire State Building, a white woman, Sarah Crandall, surreptitiously follows him to an apartment building he uses as his new home. Over the next few weeks, Sarah watches unnoticed as Ralph fixes up the building. He acquires two mannequins, whom he names "Snodgrass" and "Betsy," and talks to them. With a generator and a truck engine, Ralph lights the street lamps on the block on which he lives. When, in a fit of pique, he throws "Snodgrass" from a balcony to the pavement below, Sarah, thinking Ralph has jumped, screams. He confronts her and she explains she survived the catastrophe by taking refuge in a decompression chamber. As time goes on, the two become friends, but when Sarah suggests she move into his building, he says facetiously that people might talk. Ralph, who spends much time rescuing books from the library, advises Sarah to stay busy also. She explodes, saying she is "free, white and 21" and will do as she pleases. Upset at the remark, Ralph is further irritated when she talks about love and marriage. He tells her not to push him, then reminds her that he is "colored," a "Negro," "nigra," or "nigger," depending on who is speaking, and that in normal circumstances she would not know him because of his race. She breaks down in tears, but a few days later, Ralph brings her a diamond from Harry Winston and a newspaper headline he has printed proclaiming her birthday. That night, he plays doorman, maitre d', waiter and singer at a club to celebrate Sarah's birthday, but when she asks him to sit with her, he says he is not permitted to sit with customers and refuses her request to dance. Replying that she has pride also, she walks out. Later, Sarah calls Ralph to tell him that she has seen a boat in the East River. On it they find Benson Thacker, arriving from the Southern hemisphere in a state of exhaustion. Once Ben recovers, following a week of care from Ralph, he gets the impression that Ralph is deliberately leaving him and Sarah together. Ben thanks Ralph for the clear field regarding Sarah, but Ralph, who dislikes Ben for his condescending attitude, says that while he will not get in his way, he also will not get out of it. Peeved at Ralph, Sarah tells Ben he can kiss or make love to her, and they kiss, but she breaks away and drives off. Two weeks later, Sarah brings Ralph flowers. Ralph admits that he loves her, but when she confides that Ben has asked her to move in with him, Ralph stoically calls Ben a good man. Angry about Ralph's complacency, Sarah invites Ben to her apartment, but when he crudely suggests they have sex, Sarah declines. Unable to decide how she feels about either man, Sarah surmises she should go away alone, but Ben, saying he will make the decision for her, goes to Ralph's apartment with a gun and orders him to move on. Ralph refuses, and as he goes to meet Sarah outside the building, Ben shoots at him with a rifle from above. Ralph takes a rifle from a gun store, and throughout the night, they chase each other through the city, exchanging gunfire. At daybreak, as they reach the United Nations Building, Ralph looks up and sees an antiwar inscription written on the building. He then disposes of his gun, and after Ben follows suit, Sarah finds them. Ralph is about to leave them, saying he has work to do saving whatever he can, when she asks him not to go and puts out her hand, and he takes it. She then calls out to Ben, who takes her other hand, and the three walk together.