Ann Burden is a teenage girl who believes she is the last survivor of a nuclear war. Since her family's disappearance on a search expedition, she has lived alone on her farm in a small valley spared from radiation poisoning. A year after the war, a stranger in a radiation-proof suit approaches her valley. Afraid he might be dangerous, Ann hides in a cave and does not warn the man when he mistakenly bathes in a radioactive stream. When he falls ill, her fear of being alone forever leads her to reveal herself to help him. She discovers that the stranger is John Loomis, a chemist who helped design a prototype radiation-proof "safe-suit" at an underground lab near Ithaca, New York. Ann moves him into her house and fantasizes about eventually marrying him.
Loomis becomes delirious, with traumatic flashbacks to the underground lab. He talks of how he shot his coworker, Edward, who tried to take the safe-suit to find his family. Though troubled by this revelation, Ann nurses him through his illness and keeps secret her knowledge of Edward's death. As Loomis recovers, Ann is taken aback when he forbids her to touch the safe-suit and begins giving her orders on farming and managing resources. His explanation that they have to plan "as if this valley is the whole world and we are starting a colony," makes her uneasy.[5] Her uneasiness increases when she asks if he was ever married, and he grabs her hand roughly, rebuking her when she accidentally hits him while trying to regain her balance. One night soon afterwards, she awakes to hear Loomis in her room. When he attempts to rape her, she flees to the cave again.
Later, Ann approaches Loomis and proposes sharing the valley and farm work but living apart. He professes surprise when she tells him she won't live with him anymore and asks why, as if he has no idea. Ann remembers that he acted the same after he had grabbed her hand, "as if nothing had happened, or as if he had forgotten it."[6] She refuses to justify her choice to him, to tell him where she is living, or to come back to live with him. Loomis answers that he has no choice but to accept her proposal, though he hopes she will reconsider and "act more like an adult and less like a schoolgirl".[7] Though the arrangement is "unnatural and uneasy," and Ann worries about surviving winter, she sticks by her decision and wishes Loomis had never come.[8] Loomis locks the store, cutting off her supplies. When she approaches him for the key, Loomis shoots her in the ankle. Ann flees, realizing that he had not shot to kill, only to lame her to make her easy to capture. Loomis uses her dog, Faro, to track her to the cave, where he burns her belongings, though Ann escapes. Ann's ankle wound becomes infected. As she recovers, she has feverish dreams of another valley where children wait for her to teach them. Ann comes to believe the dreams may be true and Loomis is insane, so she plans to steal the safe-suit and find her dream valley. Moreover, she decides to kill Faro to prevent Loomis from tracking her, though she is later unable to do so. However, Faro is fatally poisoned swimming across the dead creek to her.
Ann finally acts on her plan. She lures Loomis from the house with a note offering to talk if he meets her unarmed, then steals the safe-suit and waits for Loomis to arrive. When he does, she reveals her knowledge of Edward's murder, which shocks Loomis enough to stop him from shooting her. He begs her not to leave him alone. Ann tells him she will send people to him if she finds any, and leaves. Loomis's last action is to call out that he once saw birds circling to the west. Ann walks west into the irradiated zone, hoping to see a green horizon.
