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.

Investigating officer DCI Andy Steemson asks Nikki and Jack to examine the small blood traces, bike tracks and a charred cake in the oven in the house. This was no planned disappearance.

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.

The body of 20 year-old Lorna Katz is discovered drifting near the bottom of the River Lea. Her hands are bound, suggesting she was tied up somewhere before she floated downriver.

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.

Father and son Bill and Frankie McAteer are killed in a horrific car accident on their way home from work at Billingsgate Market. Nikki and Jack arrive at the scene of the crash to meet DS Jodie Tanner, and to discover Frankie's body still in the car but riddled with bullets. Whoever was driving the SUV had a gun. Back at the Lyell centre, Nikki's post mortem examinations suggest that Frankie was the target, rather than Bill. Clarissa and Max discover a deleted video on a mobile phone showing a sexual encounter between Frankie, his girlfriend Becky and best friend Liam, which was posted on a revenge porn website. However when Liam is also killed, the video becomes the key to the killings. When Nikki and Tanner visit Becky, her flatmate Paul enters and she bursts into tears about the murder. But Paul looks like he has worries of his own… Meanwhile, Clarissa's husband Max is an expert in the fast-developing field of digital forensics and lends some equipment to help crack Frankie's se

Examining the contents of Liam's phone, Clarissa has found a longer version of the video and sees a flash of Paul's face as he peers from the stairs. Retrieving Paul's laptop, Clarissa takes it to Max's lab and finds reams of communication between Paul and another person, under pseudonyms, which starts to unravel the mystery of the Billingsgate killings. Meanwhile, Clarissa is amused to see Thomas' nose put out of joint and Jack soon feels paranoid that Max is poaching Clarissa for his own lab. Will Clarissa remain with the Lyell team?

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. Last in the series