When an eight-year-old boy finds his grandfather dead in a pub basement, he tells Rob that he was hit by a man with a baseball bat, but Vivienne doubts his story.

In exchange for turning a blind eye, Adrian gives Rob a tip about Sarah. Now believing that Sarah may still be alive, David and his team race to her find her.

With Billie stabbed and her attacker on the loose, Vivienne blames herself for not recognizing that Billie was in danger. As the team scrambles to track down Amber, they discover that there's more to her than meets the eye.

When a father and son are shot execution-style in broad daylight, the detectives immediately suspect a gangland hit and worry the violence could escalate.

A body found in a suitcase is identified as a man who has been missing for two weeks. When the team inform his widow, Cassidy, she seems unaffected and points the finger at her brother-in-law, Charlie. But Charlie claims that Cassidy is a control freak whom his brother planned to leave. Is Cassidy a cold fish or a ruthless killer? Meanwhile, David tries to convince Mulgrew to reveal what he knows.

When another black police officer is murdered, the team suspect he is the latest victim of the killer cop. After speaking with Sgt. Durrant, who attended a stop-and-search with the victim in the hours before his death, as well as with the subject of the search, the detectives suspect the crimes are racially motivated and attempt to save a high-profile target. David and Vivienne cross a huge line.

When Major Len Miller is found dead at the bottom of his stairs, a bloody fingerprint implicates his grandson; as David and Vivienne shed light on a dark secret from the major's past, they also must deal with the fallout from their indiscretion.

After young medical student Japesh Patel is stabbed to death, Billie wonders if it's connected to a similar, non-fatal attack on a gang member. Meanwhile, tensions between David and Vivienne reach a breaking point.

DC Rob Brady and Trainee DC Billie Fitzgerald respond to a gruesome scene: a man found stabbed multiple times and left hanging from a tree. DS Vivienne Cole expects to lead the investigation but is shocked when DI David Bradford takes over, as he has been on leave since the disappearance of his wife.

A delivery driver is killed, and a trace of his movements places him at a pub that his wife frequents; she has a motive, but she's an alcoholic and can't remember if she killed him or not; when David doesn't turn up for work, Vivienne confronts him.

DC Rob Brady wakes up in his car with a witness lying dead next to him and is arrested on suspicion of murder; in his eagerness to clear Rob's name, DI David Bradford interferes with the investigation against DS Vivienne Cole's advice.

A groom is killed at his bachelor party, and the man who reports the crime is the ex-husband of the bride. The detectives race to find the missing 18-year-old best man, who may have been the killer or another victim.