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.

The Gibson family are in a state of panic when Grace is discovered missing, and Marcella tries to make sense of the previous evening's events, scouring CCTV footage in an attempt to discover more about her disappearance. Cara returns home from her latest job, in which she has stolen a watch, some earrings and a bracelet, but is unaware of their significance and the danger she's put herself in, with all of the items recognisable as trophies taken by the Grove Park 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.

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

Marcella digs deep in her attempts to take down Phil Dawkins, but her dogged pursuit has tragic consequences, causing her to question everything. Meanwhile, her ongoing blackouts mean she has to face up to the threat of losing her children.

Adam gets away from the killer's prison but loses consciousness while trying to escape in the car of an elderly couple. Meanwhile, Marcella confronts Reg about a binder that was stolen from his house, while Phil sets his sights on the detective.

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.

The arrest of Vince Whitman causes massive fallout between Tim and Marcella. Meanwhile, Postfeed24’s internet viral video leads the team to discover another body much closer to home.

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.

Marcella has no time to lose as the killer has Edward. Following leads she races to the killer's house, but finds it empty except for a ringing phone. However, a premonition offers up clues, but can she get to her child before it is too late?

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.

Marcella is recognized on the street, and later finds an odd note on her windshield. When DCI Sangha arrives in Belfast, Bobby must cover his tracks.

Marcella receives an anonymous package. At Jessie's vigil, DCI Sangha spots an eerily familiar face. Finn finds out who sold Jessie the heroin.

The Maguires arrange for Bobby to leave the U.K., but the plan goes awry en route to the airport. Katherine sends a threatening message to Marcella.

Her stalker's latest threat leaves Marcella suspicious of Frank. After recognizing one of Bobby's abductors in police records, she tracks him down.

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.