Two couples -- businessman Bobby Rose and his artist wife, Carole Ridingham; his partner, Coleman Snow, and Snow's wife, Ruth Farr -- have gone on a walking tour in Wales, during which a fatal accident occurs. The question of what happened preoccupies not only an ensuing negligence trial but also the narrator, Bobby and Carole's daughter, Susan, who lives alone in her parents' house near the coast of Maine. Assisted by court transcripts, a notebook computer containing Ruth Farr's journal, and a young vagrant who has taken to camping on her doorstep, Susan lays open the moral predicament at the heart of the book: we are culpable beings, even though we live in a world of imperfect knowledge.