Wisconsin security guard Harry Cain's beloved wife Claire is murdered in a shopping mall where he works. To Cain's frustration, the local authorities have no leads or suspects, and are baffled by the presence of an additional victim, a police officer, who was shot at the same time as his wife. Determined to identify Claire's killer, Cain begins investigating the case on his own and eventually becomes so fixated that he can think of nothing else. For Cain, who continually reminds himself that his motive is not revenge, the question of "Why?" is as confounding as that of "Who?" He becomes convinced that the house across the street is somehow connected to the crime. Discovering that the house is leased to a corporate account rather than individually owned, and therefore has no name attached to it, Cain decides to break in and search for clues. Although the house is deserted, he finds a photograph of a woman and a child in front of a restaurant, and becomes convinced that the woman in the photo is somehow involved in the murder. On closer inspection, Cain determines that the restaurant in the picture is in a town in Montana, and decides to travel there in hopes of locating the woman. Upon reaching Montana, Cain begins to inquire about the woman in the photo, who turns out to be Kate, the wife of Lieutenant Peter Northup, who has just been decorated for his outstanding police work. Informed that a mysterious man from Wisconsin is looking for Kate, Peter, clearly struggling with a dark secret, grows increasingly nervous and paranoid. Unknown to all but a few close associates, Peter is a member of a secret group within the police whose goal is to clean up corruption and crime inside the force. Peter meets up with two of the group's leaders and breaks down, overwhelmed by guilt from having shot an innocent woman on his last mission to Wisconsin. The leaders order Peter to eliminate the stranger. Peter contacts Harry, promising information about the woman in the photo, but when their destinies finally cross, both men are forced to act on their gravest fears.