Two women have been murdered simultaneously but in different ways. When a connection is made, DI Thorne realizes that two serial killers are at work in a macabre partnership.
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.