Three people, the Lakhanis, an Indian couple, and an employee are shot dead by a long range sniper on a garage forecourt - a seemingly motiveless crime. Ten miles away another victim, profoundly deaf Hannah Smithson, is also shot, though her boyfriend's brother Chris, targeted with her, survives but is unable to help with any information. Inspector Jane De Freitas, on the rebound from an affair with colleague Robert Drake, calls in the pathology team but with the sniper at large and shooting indiscriminately Thomas is concerned for ex-wife Julia and their daughter Rosie, now living with former drug user Conrad Devenish. After a fifth victim, Martin Cross,a charity worker at a school is shot the killer rings Jane to tell her that he is unstoppable.

Whilst the sniper claims another body, a French lorry driver, there are fears that he may next target a school and Thomas rushes to ensure his daughter's safety. Drake calls a public meeting to pass on police progress, whereat Martin Cross's brother Ian praises him. Nikki however believes that Cross may have been the only intended victim with the others slaughtered to give the impression of a serial killer. Jack learns that Cross regularly assaulted his wife and teen-age son Craig, who hated his father, and Craig has bullets in his bedroom. Nikki and Jane now feel that Craig was working in tandem with the murderer but when the sniper strikes again Craig is standing next to the victim. They start to wonder if Martin Cross was the only person the sniper wanted out of the way and how many were working with the killer.

The team investigate the systematic shooting of a family picnicking beside a river. The case stirs memories of the death of a young woman beside that same river 10 years earlier.

As DCI Butcher moves closer to making an arrest, the team continue to pursue their own lines of inquiry. Amy's past begins to catch up with her and converge with the team's investigation.

When teacher Lucy Chatham is abducted Nikki and Jack help police officers Andy Steemson and Karen Wretzy investigate the circumstances of her disappearance whilst Thomas assists abrasive DS Maureen Steele when ex-addict Tina Lunt is murdered. Doctor's daughter Jessica Timpson saw a car outside her house near the spot where Lucy was taken and her parents Nick and Ellie fear it may belong to Ellie's brother Alec Porter. Tina's death points to criminal boyfriend Wesley Beale whilst Lucy's sister's boyfriend Jasper Lampton is the key suspect when Lucy, his secret lover, is found dead. Forensic evidence suggests that both women were killed by the same man. Then Beale is murdered.

Ex-drug user Jasper is hauled in by Steemson for killing Beale, who had been his supplier but released again whilst Ellie confides her fear to Nick that Jessica is covering for Alec, whom he believes to be the murderer. Then Clarissa finds a link between Alec and a twenty year old crime in Brighton for which he was acquitted. As a result he goes on the run and Ellie finds herself in danger from an unexpected source before the multiple killer is caught.

Lorna Katz's body is pulled from the Thames and DI Heather Ashton hauls in Kieran Vale, the boyfriend Lorna dumped but wanted to get back with. Another suspect is houseboat-dweller Aaron Logan, a boy who dated both Lorna and Ruth Tresize, a girl who disappeared three years earlier. The discovery of Lorna brings her estranged parents Ali and Chris to seek answers from Nikki. Following a clue on Lorna's phone Kieran does some sleuthing of his own, resulting in his murder, whilst Aaron, the prime suspect, goes on the run but is sought out by Ali, who attacks him.

Ali is arrested for stabbing Aaron, who discharges himself from intensive care aided by brother Ben and his wife Sarah. Visiting Ali in custody DI Ashton suggests that Chris killed Ruth and gets an angry response whilst illegal immigrant Stella Duffour is interviewed as the person who reported Lorna's death, resulting in her being threatened. The pathologists pick up on the evidence that sent Kieran to embark upon his fatal mission and locate the spot where Ruth was killed. An unusual discovery by Clarissa is also instrumental in catching the murderer and closing the case.

Clarrisa's husband Max is brought to the Lyell Centre to help crack a case.

Becky's father Stuart is arrested following the discovery of footage on Frankie's phone of Becky having reluctant sex with both the dead boys. However Clarissa spots Becky's devoted flat-mate Paul watching them surreptitiously though his DNA is not found at either murder site and he has an alibi for both crimes. Then headmaster Clive Maitland is stabbed to death, following a run-in with two ex-pupils though his murder seems unrelated to the others. When Max is recalled he establishes a lethal link between two parties with no obvious link.