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Three women are found murdered, their killer having induced them into having strokes. A fourth is still alive, and may be able to identify her assailant, except she is unable to communicate as she has been left with locked-in syndrome. DI Tom Thorne concludes that it was the killer's intention to leave his victims incapacitated, not dead.

Thorne is forced to conduct his investigation alone when Tughan dismisses his 'locked-in' theory. He and Anne Coburn try to help Alison Willetts communicate. A woman who claims to have escaped the killer is attacked.

Thorne confronts Phil Hendricks and accuses him of murdering the three girls fifteen years ago.

Two women have been murdered simultaneously near St Pancras station, but in different ways. When the connection is made with two past murders, months before but on the same day, Thorne realises that two serial killers are at work in a macabre partnership.

One of the killers is caught after a failed attempt on the life of the son of another victim. He confesses to his crimes, but refuses to identify his partner. Thorne decides to free him in an attempt to force the other killer to come forward.

Martin Palmer escapes from police custody and he and Nicklin resume their series of murders. Chen, desperate to prove herself, risks everything to catch the pair.