Arthur Seldom (John Hurt), a prestigious professor of logic who has written a recent book on the link between mathematics and serial killers, and Martin (Elijah Wood), a young graduate student who has just arrived at Oxford hoping to study with Seldom team up to solve a series of mathematically linked murders. As professor and student join forces to try and crack the code, an elaborate puzzle begins in which every action has an effect beyond anything that Martin could have imagined.