Twelve-year-old Ann Burden knows how to take care of herself. As the last person alive, she has no choice. The war and its nuclear fallout kill everyone she holds dear and, seemingly, obliterate all but the tiny settlement where she and her family once lived together. Then, one day, she sees smoke. Mr. Loomis, a man in a strange plastic suit and mask, walks into her valley. Wary yet hopeful, she secretly and hesitantly envisions marriage and restarting the human race, ideas that are quickly curbed when Mr. Loomis falls critically ill from radiation sickness. As Mr. Loomis grows stronger under Ann's dogged care, he becomes tyrannical, controlling and cruel. Ann confronts Mr. Loomis's brutal violence with relentless pacifism; then, she is forced to decide whether saving her life is worth taking his. Told through Ann's increasingly desperate diary entries, this dystopian novel sketches one possible, and ravaged, world after nuclear war.

Ann Burden thinks she may be the only person left alive in the world, in a valley untouched by nuclear radiation. She has survived alone for a year, when a scientist protected by a "safe suit" arrives in the valley. His controlling and obsessive behavior force Ann into confrontation and a crucial decision about her future.