A murderer is using his computer expertise to blackmail hospital doctors, and threatens that a patient will die every twenty-four hours unless his demands are met. Commander Blake has her own personal problems - she is being stalked by the sister of a victim of James Lampton and is stabbed in the arm, and her sister is still fighting her battle against cancer.

The police are still no closer to the blackmailer and Vera and Eileen Judd, the sisters of one of his victims, are chasing compensation for medical negligence. Brenda Thornton has managed to escape from the secure psychiatric unit and leaves a disturbing message on Commander Blake's answerphone. Commander Blake finds her sister snorting cocaine and takes it off her, hiding it in her desk at work. Brenda tries to attack Blake but Sarah gets in the way and it ends with Brenda getting burned by acid. The police discover the the wife and child of Alistair Morrison died in the same hospital as the brother of Vera and Eileen Judd. The sisters deny any knowledge of the Morrison or the blackmail but the police follow them and find that they are working with Morrison. During a police chase Eileen falls to her death.

Detective Chief Constable Blackton begins an investigation into both the Cripps shooting and the investigation into James Lampton, which drags up problems for both Commander Blake and D.C.I. Hedges. Matters become worse when Blackton begins looking into the Commander's affair and the subsequent arrest of Lampton, while she also has to deal with the death of her sister.

Blackton's investigation gets closer to the truth.