The team assists DI Paul Renick following the assumed suicide of Jamal Al Sham and discover that he was murdered and was a people smuggler, blamed by Varkey Khoury, head of a family of Syrian refugees, for killing his wife. Varkey's 15 year old daughter Akka approaches Nikki for news of her mother, leading to Nikki sheltering her whilst the police haul in another young woman Tamara as a likely killer. At the refuge camp in Calais apparent altruist Yusuf Hamed assists 12 year old Sosa to come to England whilst Kalil Aziz, an associate of Sham, is also murdered and the police and pathologists make a grim discovery in an English forest.

The pathologists join the police in removing the corpses of the refugees who suffocated in the locked van and find a possible clue in phone messages sent by the dying victims. Whilst a customs officer due for retirement is suspected of allowing the people smuggling the team establish that the culprit took money from the relatives of the migrants he then left to die, Akka's mother among them. Yusuf rescues another batch of refugees from a second locked van, escaping with Sosa before the police arrive but the real clue surfaces via a Syrian charity to which the killer donated the victims' monies.

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.

When Luisa Herrera, a former intern at the Lyell Centre, is killed in a car-jacking in her native Mexico Nikki arrives for the funeral, only to find that the body in the coffin is not Luisa's. She seeks out Luisa's boss, Dr Eva Vasquez, who runs a charitable hospital, also a centre for a dedicated band who search for families abducted by drug cartels as the police are too afraid to. Luisa was one of that group. Jack flies out to assist Nikki look for her friend, unearthing the bodies of several torture victims but chief suspect El Buitre refuses to give any information even when Nikki withholds his medication. Eva's son Ernesto is also kidnapped and Eva and Nikki, following a tip-off, go to find him. However Jack discovers Eva's car with Eva inside but no Nikki.

Frantic to find Nikki, who has been buried alive, Jack gets a mysterious phone call allowing him to ring Nikki and another call demanding a ransom and the release of cartel member El Buitre but the exchange goes wrong. Helped by Luisa's lover Gustavo Jack is able to locate and free other abductees from their underground prison, including Luisa herself and Eva's son Ernesto but Nikki is not with them. Ultimately Nikki draws on the lessons she learned from former mentor Leo to try and free herself whilst Jack discovers that she has been the victim of a desperate arrangement.