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.

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.

The perfectly-preserved body of a two-year-old girl is found in a sealed oil drum in the basement of the derelict Elliston psychiatric hospital. Commander Blake assumes the role of senior investigating officer from DI Doug James, creating early conflict within the team as they set about investigating former patients and staff. As the team closes in on two particular suspects, there is a clash with the drugs squad, who are running a parallel investigation into one of them.

Blackton's investigation gets closer to the truth.

The murder of Mrs Thornton makes Eric the prime suspect for both deaths, but conflicts with a parallel investigation into the landscape gardening business where Eric works threaten to undermine both cases.

When DCI Janet Chambers (Connie Hyde) goes into labor, Commander Clare Blake takes charge of the investigation into the death of a two-year-old girl found on the site of a now abandoned psychiatric institute. The girl has been dead for several years, but her body had been stored in a tightly sealed drum and is well preserved as a result. She also has a cross branded onto her chest. The pathologist's report is inconclusive, so the team start to check medical records of former patients.

Eric Thornton and John Littlewood figure prominently in hospital files, but when it's confirmed that Littlewood has died, the police focus all their effort on Thornton. A dark history of mental problems and violence related to them, Thornton stands out as the prime suspect in the girl's murder, but when his own mother (Celia Imrie) is murdered in cold blood, James isn't convinced that Thornton is responsible for either death.

The body of a self-made millionaire is found floating in his swimming pool two days after he was arrested by the Serious Fraud Office, having been siphoning funds to fund a lavish lifestyle with a wife and two mistresses. Commissioner Branton asks Blake to oversee the investigation into the death. DCI James soon discovers that the women knew all about about each other, and believes that each had a motive for murder.

A post mortem examination shows that Donald Griffith died from a blow to the neck, and not by drowning, supporting DCI James's theory that one or other of the women in his life had decided to profit from his death. Dutch detective Van Hauten arrives with a possible solution, and designs on Commander Blake.

A priest is brutally beaten and stabbed to death on an east London estate, where bad feeling between residents and police threatens to undermine the investigation. Suspicion falls on local boxing protege Jimmy Bannerman, but progress is hampered when Doug and Clare demonstrate very different attitudes to the brutal competitiveness of the sport. However, a vicious attack on a fellow detective forces them to form a united front. Boxer-turned-actor Gary Stretch stars as Bannerman's…

As the investigation continues, suspicion shifts from Jimmy Bannerman to Terry Donnolly when an eyewitness puts him at the scene of the murder and information on his troubled childhood comes to light. Doug has been coping with his own emotional events at home, but realises he may already know who the real killer could be.

DAC Sumpter asks Commander Clare Blake to take charge of the investigation when Joan Hall, mother of former DS Brian Hall, is found beaten to death in her own home, and Brian is accused of the murder. When his prints are found on the murder weapon and his blood also found at the scene, Clare believes it to be an open-and-shut case, but DCI James refuses to accept that his former colleague could ever be responsible for such a crime. Meanwhile, James' wife Zoe goes into labour, but an argument with a fellow-to-be proves to be the action which begins a chain of terrifying events, when his new baby is abducted from the maternity ward.

Clare begins to feel the pressure of two high-profile yet deeply personal cases coinciding. The investigation to the abduction of James' baby leads to the mothers at Zoe's maternity classes, while Brian Hall still has no recollection of what happened on the night of his mother's murder. As he becomes resigned to the fact that he may have killed her, an incident in prison spurs him to remember a crucial development. But can he persuade an already deflated James that he is telling the truth? A supervised trip to the murder scene convinces Hall that there was someone else in the room when he discovered his mother's body.

Acting on instinct, Clare digs deeper only to find that the truth is more shocking than any of them could have imagined. The investigation into James' missing baby leads Clare to Fiona George, a fellow mother at Zoe's maternity classes. However, her judgement proves to be her downfall, as she commits a desperate act in a bid to reunite James with his baby. As the consequences of Clare's actions bite, rookie DC Marion Randall's hunch leads her to the real abductor. Clare is tested both personally and professionally - but has she gone too far this time?