A suitcase is found abandoned at Heathrow Airport, and panic ensues, but it is soon discovered to be no terrorist threat. Inside is the naked body of Sofia Petrenko, a high-class prostitute, who has been strangled to death. The suitcase is traced to Vitali Malikov, a Ukrainian billionaire who admits to hiring Sofia's services, but he claims she was alive when she left him and that the case was a gift. D.C.S. Walker arrests Malikov on suspicion of murder, but later has to release him, and soon realises he has become a pawn in a much bigger game.

Another working girl, Maryna, is put on trial for assaulting Malikov. She claims self-defence but the trial is mishandled and walker is even more adamant then before that he will get the Ukrainian. Roisin, however, believes that his obsession with Malikov is blinding him to the identity of the real killer of Sonya and takes the investigation down a different route.

When well-known paediatric surgeon Jonathan Carlisle is found dead, DCI Connor uncovers conflicts stirred by a recent operation that resulted in a young girl's death. The team soon learn that the girl's father, Gary Webster, was the subject of a restraining order issued by Carlisle, and had been stalking him for months. But before he can be questioned, Webster absconds. After a tense game of cat and mouse, the police corner Webster at the zoo - but what will be the outcome of the stand-off?

The aftermath of Gary Webster’s suicide begins – a DPS enquiry, a very shaken Palmer, and a shocked but stoic Gemma, still convinced of a cover-up at the hospital. Gary wasn’t the killer; all he ever wanted was a straight answer.

Adrian Lawson offers a video recording of Amber’s surgery, showing that it was purely straightforward procedure, nothing untoward. His helpful nature doesn’t convince Connor, noting that he wouldn’t have given them something that might implicate either him or the hospital. In any case, Amber died in the ICU, six hours after the surgery. To get to the truth, they need to go back to whoever was in the ICU that night.

Walker is unhappy about what he sees as his team wasting their time investigating hospital negligence, when Carlisle’s killer has still not been found. However, Connor is convinced of a connection between his murder and the death of the Webster’s little girl. Connor’s hunch nags at Walker and he pays a discreet visit to Gemma Webster. Promising that the police are investigating the case fully and properly, and revealing that Carlisle was killed with a massive overdose of fentanyl, he asks her once more if she has any information she hasn’t revealed so far. Gemma says nothing.

With the spotlight of suspicion now falling on Lawson, Walker questions the Lawsons again. Adrian expresses shock and surprise when told that he was being implicated in Amber’s death. Adrian’s loyalty to his former boss and mentor starts to fade as, seething, he suggests that Walker look at what Carlisle was up to that night…

Terry Dyer has been recently released from prison, where he was imprisoned for a crime he still maintains he did not commit, and he returns to his East London estate in search of the truth. The father of the victim he was convicted of killing, Ken Randle, confronts and wounds him. D.C.S. Walker is empathetic to Ken's situation and releases him on bail, and then Ken soon becomes doubtful of Dyer's guilt, growing convinced that a friend of his son's was involved in his murder, and soon long-buried secrets are unearthed, leading to an unexpected twist, while D.S. Satchell finds his career may be in danger.

D.C.S. Walker goes on leave back to Glasgow, where he is taking care of his increasingly senile mother, he soon becomes involved in the case of a missing woman, who is the sister of an old friend. Walker begins to delve into the life of her then husband Kevin Reid, and discovers a dark bond between him and his brother Ronnie. Soon a body is discovered and is it is proved to be the missing woman. Then Kevin's second wife is reported missing - convincing Walker that one or both of the Reid brothers are involved in the crimes.

Whilst Anna's diary entries initially suggest that Ronnie was stalking her, they are eventually dismissed as forgeries. Ronnie also had an alibi for the time that Margaret went missing and it looks as if he is being framed, possibly by his brother, with whom there is an explosive showdown at a race course.

The body of a teenage girl, Maria Cole, is discovered in a chalk pit, and the tracks of three different vehicles are discovered at the crime scene. They eventually lead D.C.I. Connor and D.C.S. Walker to the local hardman, Ray Harper. His son Andy and his friend Darren admit being at the pit on the same night, though they claim that they never saw the girl there. However, cracks soon appear in their story when CCTV footage reveals that they crossed paths with Maria before her death, later leading to the two boys being charged with her murder.

Ray Harper is devastated to find that Andy has hanged himself. Darren admits that he and Andy were a gay couple but dared not tell their families. The case against him collapses and he is released. Mike Walker is having troubles of his own with his teenage son but when Roisin decides to visit Darren at his flat she is in for a shock.