Set in London, each episode is a self-contained story, starting with a news report, then following the team of three detectives as they investigate the circumstances the crime. The cases themselves are hard-hitting with contemporary themes, such as the search for a soldier with PTSD, a murder that has been made to look like an assisted suicide and the gang rape of a young teenager.
After an early release from prison due to cancer, Dean Price is beaten, stabbed and left for dead in a copycat crime, identical to the one he was convicted of having a part in eight years ago. The father of the original victim comes under suspicion, after reports of an argument between him and Dean and evidence of blood on Dean’s kitchen floor. In an effort to get to the bottom of what happened, the east London detectives decide to reopen the original investigation.
The team are shocked by the shooting in broad daylight of a popular clergyman outside the boxing club that he runs for disaffected youths. Martha suspects that the vicar’s hug-a-hoodie approach has backfired - but the investigation soon moves into far darker territory…
The detectives are met with a wall of silence when they try to find out who was responsible for the brutal attack of an elderly widow in her home on a notorious London estate. The victim claims her life savings of £10,000 were taken from her spare room, but the team doubts her veracity. The investigation points towards numerous suspects, including the victim’s son, a man with convictions for robbery and cannabis possession, and another with a history of violence.
Ken Baxter stages a roof-top protest over his missing soldier son Pete whilst Pete's mistress Sophie is the victim of a hit and run incident. Pete's friend Eddie is a primer suspect as is Sophie's husband Ricky whilst Sophie's friend Caitlin also has something to hide. Ricky is arrested for the hit and run but when Ken is also killed Martha and her team must look elsewhere for the guilty party.
The unit is stunned when DI Martha Bellamy is murdered in her own home. DCI Daniel Drummond leads the investigation, bringing in officers from Major Crimes to help - but someone knows more than they are letting on.
A new murder victim is linked to the Bellamy blackmail case forcing Jack to pressure his old flame into testifying against her boyfriend, who Jack thinks is responsible for the blackmail and Martha's murder.
When a girl is raped on her way home, Jack tries to conceal his connection to the key witness in the case, Lucy. The 16-year-old has been living with him since he discovered that she is probably his daughter, and Charlie reluctantly agrees to keep his secret. However, it soon becomes clear that both girls are lying about that evening, and when more evidence emerges, Charlie cannot stay silent for much longer.
Jack finds it impossible to sit at home on the sidelines when his daughter is abducted.
The body of Sarah Kramer, who went missing eight years earlier is found and Jack interviews her widowed father Joseph, who tells him that Martha had been investigating the disappearance. Retired officer Ed Goddard, no friend of Drummond, reports to Gary that Sarah was an informant and Drummond her handler and secret lover whilst Jack also lets Gary in on Drummond's confession regarding Stan's death. Drummond covers himself to the team, throwing suspicion on Mo as the object of Sarah's investigation and her killer, to which he admits but adds that Drummond buried her ...
Alisha obtains evidence to incriminate Drummond from his wife Chrissy and, after she is attacked and it is stolen, its recovery proves that Drummond's son Luke was the attacker. Realizing that the game could be up for him Drummond goes to see Joseph and asks him to keep a secret. The connection between Joseph, Mo Jones and Drummond will eventually solve the mystery as to who killed Sarah, and Martha and her husband and bring Drummond down.