The story follows a young mother who, when on a day trip with her husband and son, commits a startling act of violence and, to her horror, has no idea why. The event launches an inverted and utterly surprising crime thriller whose driving force is not the "who or the what, but the why". When an investigator finds himself obsessed with uncovering the woman's buried motive, together they travel a harrowing journey into the depths of her psyche.