London police detectives Cassie Stuart and Sunny Khan investigate historic cold cases involving missing persons, murder and long-hidden secrets.
A dismembered body is found in a scrapyard and the team believes the remains had been stored in a domestic freezer for 30 years. Meanwhile, after making the decision to retire from the police force, Cassie faces a dilemma when she learns she isn't entitled to her full pension payment unless she completes her 30 years of service.
Cassie and Sunny identify four potential suspects who trained as police probationers together, and two of them are still serving officers. They have no luck finding the rest of the body and decide to go public about the discovery of Walsh's remains.
Cassie and Sunny interview each of the four suspects, who all deny knowing the victim. Bouting discovers Walsh was cautioned three weeks before his death.
Cassie and Sunny discover Walsh was chased by all four suspects on the night in question. Willets realizes Fiona's blood sample may have been deliberately lost by Liz.
Cassie and Sunny interview two of the suspects again and get closer to the truth of what happened to Walsh. Balcombe believes she may have found the cause of death.
Despite a tragic turn of events, Sunny and the team narrow down the suspects.