"Z for Zachariah" is a post-apocalyptic novel written by Robert C. O'Brien. The story follows Ann Burden, a young girl who believes she is the last survivor in a world devastated by nuclear war. She lives in a secluded valley that has somehow remained untouched by radiation. The plot thickens when John Loomis, a scientist, arrives in the valley. Initially, Ann is excited to meet another person, but she soon realizes that Loomis poses a threat. The novel explores themes of survival, trust, and the moral dilemmas faced in a changed world.