What if the only way you could prevent a murder was by confessing to it? Jemma Stelling has confessed to a murder. She is the police's lead suspect. She couldn't have committed the crime. She has an unshakeable alibi: at the time of Marianne Cass's brutal murder, Jemma was at the police station, confessing to a murderous obsession with Marianne. Is Jemma Stelling innocent? Or is she an ingenious, cold-blooded killer? Can you be guilty of the opposite of murder?