Ann is a resourceful 15-year-old survivor who maintains her family's farm in a radiation-free valley following a catastrophic nuclear war. Having lived in total isolation for over a year after her family left to search for survivors, she relies on her practical knowledge of agriculture, animal care, and the natural world to sustain herself. While she deeply desires human connection, she is acutely protective of her physical safety and independence.

John Loomis is a former military chemist who worked in Ithaca, New York, developing protective plastics before the war. He arrives in the valley exhausted and soon falls critically ill with radiation poisoning after bathing in a contaminated creek. Highly intelligent and scientifically minded, he approaches survival through engineering and efficiency, directly contrasting with Ann's reliance on nature.

Edward is a scientist who worked alongside John Loomis and Dr. Kylmer at the laboratory in Ithaca. He helped develop the experimental radiation-resistant suit that Loomis eventually uses to travel to the valley. Desperate to find his wife and son after the war, his personal priorities clash dramatically with Loomis's strictly logical approach to survival.

Faro is a dog who originally belonged to Ann's cousin, David. He manages to survive the wasteland outside the valley and returns thin, irradiated, and missing patches of hair. Because he possesses an excellent sense of smell, his ability to track human scents introduces an unpredictable dynamic to the isolated valley.

Joseph is Ann's brother who lived with the family in the valley. When the adults set out on an expedition to search Ogdentown and the surrounding Amish communities for other survivors, they order Joseph and Ann to stay behind for safety. Defying these orders, Joseph hides in the back of the expedition truck.

David is Ann's cousin who lived with her family before the war. He joins Ann's parents and their neighbors on their search for other human survivors in the surrounding destroyed towns.

Ann's father was a pragmatic farmer who understood the land and agricultural cycles intimately. Following the brief nuclear war, he leads the adults of the valley on a search mission to nearby towns, inadvertently leaving Ann as the sole caretaker of their home.

Ann's mother lived in the valley and joined her husband on the search mission to nearby Ogdentown. She appears in Ann's dreams, bringing her comfort and helping her recall details about the natural resources of the valley, such as where to find watercress.

Dr. Kylmer is a scientist who worked at the laboratory in Ithaca, New York, prior to the war. He collaborated with John Loomis and Edward to develop radiation-resistant plastics and safe-suits for the United States Army.