Former detective Marcella Backland is shocked to the core when her husband Jason leaves her unexpectedly, confessing he no longer loves her. Heartbroken, she returns to her job on the Met's murder squad that she gave up 10 years before to have her second child, and decides to follow up on a hunch she had in a case involving a serial killer.

Grace's body is discovered by the police and Marcella tries to keep her head and hide the fact that she was one of the last people to see her alive. Sylvie is distraught at the loss of her daughter and her dismay is further compounded when she learns that Grace and Jason were having an affair, while Henry tries to find comfort with his friends Matthew and Yann. Meanwhile, Peter is released from open prison and makes a beeline for Maddy, who lies to him about having a boyfriend.

An aborted attack on an elderly man gives the police a new lead in the Grove Park killings, while Marcella tries to track down the taxi driver from outside Grace Gibson's house, fearful that he saw her on the night of the murder.

Mo holds the police responsible when Hassan is shot, while Marcella triumphs over Peter Cullen and finally gets the information she is after relating to Grace's death. Jason manages to get his job at DTG back - but at what cost? - and deliveryman Ronnie makes the mistake of his life.

The killer's next victim is a six-year-old girl, which devastates Marcella, and a DNA match gives the police team its strongest lead yet. Meanwhile, DI Tim Williamson investigates Andrew Barnes' death - was it really suicide?

Mo confronts Marcella after finding his brother dead, while the detective has her doubts as the police continue to investigate Yann. Tim starts to look into the curious circumstances surrounding the death of Andrew Barnes, bringing him into contact with Jason.

In her desperate hunt for Matthew, Marcella questions Henry at his home. Meanwhile, the pressure mounts on Jason as Tim's investigation into the murder of Andrew Barnes gathers pace, and the killer is finally revealed, causing shockwaves among the entire team.