After off-duty police officer Sgt. Ian Durrant finds a dead man at the wheel of his van, the detectives link the victim to a woman who was killed while in police custody. As DC Rob Brady was the one who discovered her dead, the case affects him deeply. When Trainee DC Billie Fitzgerald finds a connection to a murdered young cop, DI David Bradford suspects the killer could be a police officer.

When schoolboy Luis Moretti is found dead in a lake, the detectives learn of a potential connection to the recent suicide of a young girl. Meanwhile, DS Vivienne Cole tells David that she believes Detective Chief Superintendent Mulgrew was behind the threat to Rob's family, and she reveals her past relationship with the chief.

After a locksmith is murdered, Billie recognizes the victim and learns he was a witness to her father's death 10 years earlier. The investigation brings her face-to-face with her father's killer and forces her to wrestle with her personal and professional ethics. Vivienne pleads with her ex-husband to do the right thing, but he is afraid they're both in danger.

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.