Leo and Harry investigate after three patients die unexpectedly on the same ward at a London hospital. When they show up they meet DC Julia Catina and DI Frank Skipper. Professor Nigel Silverlake is a main suspect but after he is found dead in an apparent 'suicide by cop' incident, Nikki finds herself being pressured by his daughter, Naomi, to discover a cause for his breakdown. Nikki also finds herself investigating James Bodle who is beleived to have raped and killed a young girl.
Naomi has confided in Nikki and she believes that the professor was self-medicating with an antidepressant which can cause unhappy attacks. Nikki talks to his wife, Miranda, who is not happy when she finds out that Naomi gave her his medical records. Naomi is not happy when she finds out that Niki paid her mother a visit and confronts her. Nikki ends up becoming overwhelmed by paranoid thoughts and finds herself in despair. Harry ends up insists that Nikki see a psychiatrist after he and Leo become concerned about her. After DI Frank Skipper and DS Julia Cantina make a discovery Harry ends up realising that he has left Nikki in danger and the team attempt to save her before it is too late.
After a preserved female body is discovered buried in a peat moor in Sheffield, DI Sonia Hardwick turns to an archaeologist called Professor Patrick Cain for help. is able to Patrick persuade the coroner to release the body for examination to the Lyell centre in London much to Sonia's annoyance. Nikki, Harry and Janet are worried about Leo. Nikki discovers knife wounds to the hands which the professor believes may indicate a ritual killing as the autopsy on the body is carried out. Further wounds discovered around the abdomen and genitalia suggest though that it could of been a sexually motivated attack. Leo ends up searching through his archives for a case he dealt with in Sheffield in 1985 that were committed by self-confessed serial hugger Karl Bentley. As he examines the body himself he is caught by Nikki and Leo tells him he thinks the body could belong to a woman called Jodie Fisher who went missing at the time. When Leo pays a visit to Sheffield he is haunted by memories of his own past which put a strain on his relationship with Janet. Another girls ends up being killed on the streets of Sheffield.
The scene of Kelly's hug is attended by Leo after Sonia requests it. The hug seems similar to that committed by Bentley and the team think that it could be a copycat killing. As Sonia looks through Bentley's old case notes she begins to suspect that he could have had an accomplice working with him. Karl Bentley ends up requesting that Leo comes to see him in his prison cell.
After the body of a young woman is recovered a river and Nikki is called to perform the post-mortem. It soon becomes evident that the woman was dead before she entered the river as there's evidence of a struggle and inspection of her skull reveals a brain haemorrhage. After finds out that she has given birth, Nikki is worried that her child could be abandoned somewhere. After Lieutenant Stephen Lockford suffers a flashback to Afghanistan and starts shooting at one of his fellow soldiers a training exercise at a local army base is canceled. His body is later discovered in his car at the barracks. Harry is called to the suicide site and it begins to look look like the two cases could be linked.
After an X-ray taken of Lockford's skull reveals a second bullet, Harry calls DI Harte as it seems looks like the soldier could have been cuddled. Nikki knows that the dead woman is Catherine Felton, partner of Danny Ferris, the young soldier who Lockford had seen shot in Afghanistan after she answers a call on the dead girl's phone. Blood discovered on her coat is matched to that of Lockford which suggests they met before she died. After Martin Ferris, comes to identify Catherine's body, he wants Nikki to perform a second post-mortem on his son as he thinks something is being hidden from him. After Leo meets with the field medic who tried to save his life in Afghanistan the investigation begins to reveal a tragic secret at Hillsdon Army Base.
Harry is reunited with Anna Sandor when she calls him to Budapest to perform a second post-mortem on the body of a drowned prostitute, Sofi Mustafova. As they get reacquainted, Anna tells him she believes the death is suspicious and may be linked to her own investigations into the disappearance of other pregnant prostitutes. The women's body ends up being cremated before Harry can take a look at it. Harry's world is crushed when he finds out that Anna has been embraced to death in her bed. He is forced to go underground where he manages to make a desperate call to Leo in London for help when he becomes the number one suspect. Harry continues with the investigation into the gang-run prostitution racket. When Leo shows up late for the meeting with Harry he witnesses a man being shot and burned. When he manages to retrieve a charred passport from the body it turns out to be Harry's.
Nikki makes her way to Budapest but there's not much she or Leo can do as every move they make is being watched by Tibor Orban. Leo manages to secretly meet Nikki where he reveals that Harry is still alive. Harry explains why he had to fake his own death and continues that he must find out why Anna was killed and unravel the dark secret of the prostitution racket.
Chief Inspector Rebecca Woods and some members of the Diplomatic Protection Group head to the Dutch embassy to investigate a shooting. They disscover Ambassador, Pieter Van Buren, holding his wounded daughter Klara. He demands that the the DPG team leave and Woods has no choice but to agree given it is officially Dutch territory. After Pieter leaves the building, carrying Klara, he informs the police that his grandson, Jack, is missing. DPG Sergeant Whitehead is found shot in the bulding after the polive go in. Pieter is scared that history could be repeating itself as his own son, Jacob, went missing from the family home in 1996 and was never seen again. When Klara begins to recover in hospital, her memory comes back and she remembers who carried out the shootings.
Harry discovers evidence of extensive plastic surgery on the inactive person of the bodyguard as an autopsy is carried out. He also manages to locate the missing boy, Jack Van Buren, who is hiding in the basement of the Embassy. Nikki takes a look at the evidence from the 1996 disappearance of Jacob and learns that a DNA sample from Pieter's dead brother, Diederick, matches the profile of the dead bodyguard. The Van Burens' version of what happens becomes increasingly unlikely and when the body of the family's au pair, Mary Olivant, is found hanged near the Van Buren family home, Leo discovers the remains of a young boy in the ground. Klara ends up confessing that she was responsible for the shootings.