The story follows a girl, Ann, who thinks she is the last human alive after a war devastates everything beyond her small valley. But then a man appears. Ann isn't sure whether to trust him at first, but when he succumbs to radiation sickness, she takes care of him. Once he's better, Ann realizes that maybe she would have been better off alone after all; the man turns controlling and may have suffered too much during the war to have kept his sanity.
This story is so taut and fast-moving that once I sat down with it today (having started it yesterday) I had to read to the end. At the same time, it wasn't so scary that I couldn't take it (I'm kind of a wimp when it comes to scary stuff). The title, which I thought was a bit strange, refers to a picture book Ann remembers reading when she was little, which started off with "A for Adam" and ended with "Z for Zachariah." Ann figured since Adam was the first man, Zachariah must be the last.