After her marriage dissolves, former detective Marcella Backland struggles with jealousy and anger when she returns to work to hunt down a serial killer.

Marcella is in turmoil as she tries to piece together what unfolded at Grace's, as the team continue to hunt the killer.

The team discover Marcella's ties to Grace; Jason's affair is revealed; Peter remains a focus in the murders.

Marcella tries to find the taxi driver that was outside Grace's house on the night of the murder. Meanwhile, there is a new lead in the Grove Park case following an attack on an elderly man.

Jason manages to get his job back, while Marcella triumphs over Peter Cullen and finally gets the information relating to Grace's case.

The killer targets a six-year-old girl, which devastates Marcella. A DNA match on the corpse gives the police their strongest lead yet.