Traveling salesman Harold Speck is approached by a man in a diner who asks him an uncomfortable question. After leaving the diner, Harold is found dead with his eyelids cut off and clutching a symbol consisting of a circle with a line through it. The murder is investigated by Federal Bureau of Investigation Agent Thomas Mackelway (Aaron Eckhart), who was recently suspended for beating suspected serial killer Raymond Starkey. Mackelway receives a series of taunting faxes from someone who may be Speck's killer. As the investigation proceeds, Mackelway and his partner, Fran Kulok (Carrie-Anne Moss), become aware of the possible existence of Suspect Zero, a "super serial killer" responsible for hundreds of deaths who leaves no evidence behind to link his crimes together.