Following the suicide of colleague Lizzie Fraser, who he believes was scapegoated by police for a wrong decision, Leo comes to work in an angry, bitter mood. DI Connie James brings Leo, Harry and Nikki in following the brutal slaying of ex-SAS soldier Mick Francis, his daughter Jessie, and her young son Luke at their farm shop during a robbery. Harry establishes a good rapport with the pregnant Connie but then Superintendent Byrne links the murders to that of Lizzie's sister Mary twelve years earlier and to several other unsolved murders in the intervening years. Byrne believes the killer is a drug-addicted woman known as the Wraith, working with a male accomplice. However unknown to him a hooded white boy who was at the shop would seem to fit the bill.

An eye witness account and identification of a car links a youth called Lee Ness to the farm shop murders but he claims not to know the Wraith. Tom Byrne,recently widowed,and Nikki,who has just lost her father,develop a rapport whilst interviewing the parents of another supposed victim of the Wraith,who have their own views on the murderer's identity. A discrepancy in the findings of the Wraith's DNA at different crime scenes leads Harry to take samples from a rival laboratory. The result reveals an elaborately plotted revenge scenario to punish the police for the deaths of Mary and Lizzie.

When Nikki sees barefoot teen-ager Shannon Kelly run over by a car whilst fleeing somebody she sits with her until the ambulance arrives and later visits her in hospital,where it is clear she is frightened of a man. DS Masood tells Nikki Shannon has been sexually abused and suspects Abdul,her estranged stepfather. Shannon turns up at Nikki's and reluctantly agrees to go back to hospital. Meanwhile a gang of young Asian men are picking up and grooming white school-girls for the sex trade. They already have Shannon's friend Amy and go after two more,Hannah and Lauren. The pathologists investigate two seemingly unrelated cases - the remains of a young girl found in discarded airport luggage and a heavily tattooed man,John Dear,who died of toxic poisoning. Then a third corpse is found - that of Amy.

Through a DNA link the pathologists find that John Dear was a client of unwilling child prostitutes Shannon and the deceased Amy whilst the suitcase victim is also identified. Nikki visits Shannon in hospital and learns about the gang of abductors but the young girl is too scared to name names. Through CCTV DS Masood gets the car registration number of a visitor to Shannon and arrives at a taxi firm where Salman,a somewhat reluctant and kind-hearted gang member, works. Salman helps Hannah and Lauren escape but is killed when his taxi crashes. The three girls are returned home but will not speak about their ordeal and,with Abdul swearing revenge, Nikki must help DS Masood resolve matters. The case closed Harry announces that he is leaving to take up a professorship in New York.

Leo gives evidence at a custody hearing in the wake of a child's death,his opinion over-ruling that of Dr Helen Karamides,which annoys her former pupil Nikki,who sees Helen as the victim of the male establishment and cannot believe the accusation that Helen is retaining tissue samples from past cases. Nikki is also approached by Andy Farmer. His mother Annie was the appropriate adult at the interview with serial killer Arnold Mears,who now directs Annie to the sites of his victims' remains where she collects their bones. Andy wants Nikki to help identify them. Nikki also discovers that Helen had a professional involvement with Mears.

Leo feels guilty when Helen kills herself,leaving an explanation on Camcorder that she took the tissues to further scientific research. Aware that Mears killed more women than he was imprisoned for, Leo tells the police about the arrangement with Annie,scaring Andy, who believes that Mears will have his mother murdered for the breach of trust. It is also revealed that Annie believes her daughter was one of Mears' victims,hence her involvement with him. When Annie disappears Nikki visits Mears,who agrees to tell her about all his other victims. He appears to have won his mind game with her but his possessiveness over his copy of 'Paradise Lost' and Nikki's chance remark about an elderly lady she met who knew Mears yield results.

Harry's work colleague Justine Thompson is murdered in her house with her daughter Gemma whilst her stepson Charlie, also attacked, lies in a coma. Baby daughter Ellie, however, is unharmed. Husband Stuart is prime suspect but he has an alibi and his grief seems genuine. The post mortem shows that Gemma was pregnant and the team learns that Justine once needed a restraining order against Stuart's brother Kevin, who was obsessed with her. Mrs Barron, the lady from next door who found the bodies, finds evidence on her son Joel's computer that Gemma and Charlie were lovers as Joel rides off to see Kevin.

After Kevin dies in a motorcycle crash Harry is surprised to see Joel keeping a vigil by Charlie's hospital bed. Whilst denying murder Joel professes that he loved Gemma was shocked that she was sleeping with Charlie. Stuart's initial alibi breaks down when CCTV footage shows Kevin, not him, at the hotel in question but he admits to an affair with Mrs Barron. The murderer is eventually exposed after a shocking betrayal is revealed, leading to an horrific reaction.

Prison inspector Rachel Kruger is murdered,along with young Nick Owen,a man she was visiting,whose computer is stolen.Inspector Bridges tells Leo she reckons Rachel's husband Peter killed his wife and her suspected lover but Peter claims that Rachel was in danger because she was investigating sinister events at Redhill prison,particularly the mysterious death of convict James Wade,and Leo believes him. The team joins the police investigation of Redhill,where governor Cairns is less than helpful and Rachel's original source,Dr Ross,too scared to speak out. But, unbeknown to the pathologists, Bridges knows far more than she is letting on.

When prison officer Ellis Roberts and convict Benjamin Johnson are killed after a supposed cell fight Leo is suspicious of corrupt officer Kessler,whose evidence about the fight does not ring true and is found to have fathered a child with Andrea Bridges before he left the police under a cloud. Nikki persuades Bridges to confess that she covered for Kessler in the past but,despite Kessler visiting Leo to threaten him,there is doubt that he murdered James Wade and the prison inspectors Kruger and Owen. The eventual discovery of the truth spells bitterness for both Leo,ending his relationship with Janet,and Harry.

Leo is called in by his old college friend,psychiatrist Sean Delaney,now a cancer-stricken alcoholic. Sean was treating inspirational,deeply religious teenager Eve Gilston,who felt neglected by her family ,and the verdict of whose death,alone in bed,apparently of arrhythmic death syndrome Sean does not believe. Accompanied by down-to-earth sergeant Brooks Leo interviews Eve's parents,lapsed Catholics Beth and Tony,wondering if the family is prone to the rare heart condition Long QT Syndrome,though John,their blind younger son refuses to be tested. Sean tells Leo that,at their last session, Eve believed that she was possessed by evil and soon afterwards Leo and sergeant Brooks find evidence that a priest performed an exorcism on her.

Shocked by the news of the exorcism Sean joins Leo in visiting Father Jacobs though he tells them that he did not perform the exorcism and Nikki tracks down the nun,Sister Landsley,who did so,only to find she has killed herself. Leo believes the trauma of the exorcism triggered Eve's death but then he learns from Beth how Eve thought that she was possessed after passing on the measles which blinded him to John when they were younger. With the knowledge that his grandmother killed herself John believes that possession runs in the family and now he is host to the demons, leading to a further exorcism.