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.

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.

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.

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?

Kevin Hoffs and his blue lorry are found abandoned in a lay by. Leann informs Marcella that the lorry had been there for 36 hours. After analysing Adam's phone records, the team become aware that Adam had contacted the KidsCall helpline. Marcella heads there to retrieve the records and Maya is helpful despite both women having Tim Williamson in common. From Adam's records they see that he'd admitted to being abused by his father but his father has alibis for many incidents surrounding the case.

Marcella heads to Nigel Stafford's house to question him. As she walks around the house she finds a small grave before heading inside the unlocked door. Inside she's confronted by Nigel, holding a knife.

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.

Marcella recalls how Frank rescued and reprogrammed her 18 months earlier. When Stacey flees from the Maguires with her baby, Marcella must find her.

Finn uses Rory's phobia to bully him into an alliance. Marcella confronts Frank with a discovery about his past. DCI Sangha learns of Stacey's arrest.

Marcella's latest blackout leaves her with an unexplained corpse. Wondering why DCI Sangha is still in Belfast, Jack decides to investigate on his own.

The discovery of a young boy's body sends Marcella and the team after a child killer.

Eighteen months later, Marcella has a new life as a deep undercover operative in Belfast, spying on a powerful family that's running a shady business.

Dr Fairclough tells Marcella that it might be a good idea to go back to the root cause of her blackouts - Juliet's cot death. But when she tries to relive that moment at her home, the resulting stress levels put her into another fugue state. Meanhile, the team is called out when the driver of a car abandons his vehicle after a fatal collision with a cyclist and police attending the scene open the boot to discover a body surrounded by the same stuffed toys found with Leo.

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.