After 11 years’ absence away from the force, DS Marcella Backland finds herself drawn back to police work following the return of the Grove Park Killer, her last case before she left to start a family. In 2005, the killer tied the victims’ hands and feet with cable ties before taping a plastic bag around their heads. The case went cold. But now, over ten years later, it seems he has returned.
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.
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.
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.