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.

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

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.

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.