Troubled San Francisco homicide detective Valerie Hart is planning a rare weekend away from the job when she gets the call. A body has been found. A woman, brutally murdered. And the cryptic note left by the body is addressed to Valerie.

The victim is unknown to her, but as Valerie analyses the scene, the clues begin to point in a deeply disturbing direction: to a maximum security prison where a woman called Katherine Glass is awaiting execution for a series of gruesome killings. And Valerie was the cop who put her there.

The last thing Valerie wants to do is reenter Katherine's twisted world, but when a second body is discovered, with another puzzling clue, she realises she has no choice.

Katherine Glass holds the key to the killings, and Valerie needs to find out what she knows before the murders come even closer to home. Even if it means playing a deadly game where once again the psychopathic killer holds all the cards.