Three women are found murdered, their killer has induced them into having strokes. A fourth victim is still alive, but she is left with locked-in syndrome.

Thorne is forced to conduct his investigation alone when Tughan dismisses his "locked-in" theory. 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.

One of the killers is caught after a failed attempt. He confesses but refuses to identify his partner. Thorne decides to free him in an attempt to find the other killer.

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