Sam Reed can travel back in time, and makes his living helping to identify killers to the police. The problem is that if changes anything, deliberately or accidentally, the "butterfly effect" causes history to change. When he returns to the present, things are often completely different and he has no clear memory of what happened in his new history.
His troubles begin when he "breaks the rules" by trying to help the sister of his murdered girlfriend find out who the murderer was, and ends up changing his own history. Every attempt he makes to fix the problems that occur create even worse problems, and each trip scrambles his memory even further and puts him in an even worse situation than before.