Commander Clare Blake has risen to the top of her profession in the male-dominated Metropolitan Police Service, and is now the head of the serious crimes unit. Some years previously, she arrested James Lampton (Hugh Bonneville) for the murder of his beloved girlfriend. He was subsequently sentenced to life imprisonment, but has since been released on parole and has now become a successful author with a best-selling book based on his time in prison. Blake believes that Lampton is a reformed character, and begins to warm to his charms. However, Blake's colleagues, especially DCI Mike Hedges, believe that Lampton is responsible for another murder - and set out to prove his guilt

Things become complicated when Blake and Lampton become intimate, and the question arises whether he is being honest with her, or whether he is simply a master manipulator. Meanwhile, DCI Hedges and his team find themselves under investigation as Blake and her murder review team investigate the police shooting of Lionel Cripps, who was shot to death by an SO19 team following an armed stand off at his house. When Blake discovers that Cripps was a big-time police informant, she and her team set out to find out who is really responsible for Cripps' death.

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.