When a people smuggler is found dead in the City, Nikki must delve into London's illegal immigrant community. She meets a vulnerable Syrian teen called Akka who is in need of help, putting Nikki in a moral dilemma - and the killer is still on the loose.

When Nikki, Jack and Thomas make a horrific discovery in the van, they are confronted with the work of a killer who preys on illegal immigrants. With the help of Clarissa, they must work out where the killer will strike next before more vulnerable people meet the same fate.

When a photogenic young schoolteacher goes missing, Jack and Nikki are called to the scene and quickly find themselves at the centre of a media storm. Meanwhile, Thomas fights a lone crusade for justice for a troubled young woman who was killed in puzzling circumstances.

Having found a link between the two cases, the team struggles to make sense of it. Nikki visits a disgraced former colleague who may hold the key. Thomas uncovers the bizarre truth about how Tina died.

A woman's body found floating in the Thames leads Nikki and Jack to a community on the banks of the River Lee. Three years ago a teenager called Ruth Tresize went missing from the area, leaving the locals haunted by suspicion and grief. A fresh spate of killings reopens these old wounds.

Nikki is under fire for leaking information on a suspect, and Thomas is called in to play peacemaker. Jack and DI Ashton track down the mystery witness. Before the killer strikes again, the team must uncover the true story of Ruth Tresize's disappearance three years earlier.

Nikki helps Becky come to terms with her trauma. Jack is paranoid that Clarissa will leave the Lyell Centre. Paul is in way over his head.

Nikki travels to Mexico to attend the funeral of Luisa, a former intern at the Lyell Centre who was murdered in a carjacking in her native country. She soon stumbles across the post-mortem report and realises that the body is not that of her ex-colleagues. Assisted by Dr Eva Vasquez and fired up with the idea that Luisa might still be alive, Nikki and Jack trace her last-known movements and are led to a farm where a cartel massacre was carried out.

A frantic Jack carries out forensic examinations on the spot where Nikki disappeared. With Thomas at the airport ready to come out and help, but still 24 hours away and no leads left to follow, Jack receives a text message containing a mysterious phone number. However when Jack calls through, expecting to hear from Nikki's captors, he is shocked at who answers the phone.