A mother lies brutally murdered in her ransacked, two-bedroomed flat. Beside her are the bloodstained bodies of the two police officers who were first on the scene. In the bedroom are the victim's two children, a six-year-old boy and a baby, slaughtered as they slept.

And the killer is still in the flat.

Paterson sets off in hot pursuit. But after a desperate rooftop chase, the killer escapes.

The following day, another woman is found dead in her own home, her decapitated body carefully posed in her living room, her wrists pierced by crucifixes.

Why were these women targeted?

Paterson and Clocks have never had to deal with a killer like this. A killer who shows no mercy. A killer who can't be reasoned with. A killer who covers their tracks with super-human efficiency.

If they are to get their man, the two detectives must break every rule in the book.

At the same time, married life is not turning out as Clocks had hoped, and his newly-discovered son has just given him some very bad news . . .