Detective Sergeant Nolan Cormac believes he has closed the book on serial killer David Burnham, but his certainty is shattered when he receives anonymous letters insisting that Burnham is innocent of one of the five murders he confessed to—that of Marie Simpson.

The letters provide cryptic but accurate clues that force Cormac, against the objections of his superior DI James, to unofficially reopen the case. Guided by the mysterious correspondent, Cormac uncovers a key witness, and damning CCTV evidence that suggests Burnham couldn’t have been Marie’s killer.

To solve the riddle and uncover what really happened Cormac is faced with the unsavoury task of visiting Burnham in prison once more. In a series of tense interviews Burnham is surprisingly helpful and Cormac discovers who really did kill Marie Simpson and why.

But behind it all lies a deeper conspiracy that reveals Burnham's true motive …