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.