In the not-distant future, a disastrous radioactive event has obliterated most of civilization. Yet somewhere deep in the American South is a valley that remains untouched. A young farm girl, Ann Burden, is grittily surviving on her own in this valley, fearing she may actually be the proverbial last woman on earth. Then, she discovers the most astonishing sight of her life: another human being. John Loomis is a distraught scientist nearly driven mad by radiation exposure and his desperate search for others. He and Ann share little in common beyond survival. As Ann nurses Loomis back to health, they build a strand of trust across the gulf between them. With all the time in the world, they imagine forging a future - for themselves, for humanity. But when a stranger enters the valley, their precarious bond begins to unravel. Now, all three must grapple with an unstable situation - one of secrets, suspicions and mortal threats.