Robbie Coltrane stars as Fitz, a brilliant but flawed, wise-cracking criminal psychologist who looks behind the evidence of murder to discover what lies within the criminal mind. Homeless factory worker Bill Nash (John Simm) is given refuge by his foreman Stuart Grady (Liam Cunningham), but the landlady gets into an argument with Bill, and he murders her. After being interviewed by the police, Bill and Grady go on the run which leads them to Nash's adoptive family's home and yet another death. The police make the connection between Bill and Grady, who is in hospital after getting burnt. With Grady captured, Fitz realises where Bill will go next but can he make it in time to save Diane Nash (Annette Ekblom)?