When the body of a teenage girl who went missing at the turn of the millennium is found at a building site off the M1 motorway, four men are placed under the spotlight and their relationships are tested to the limit.

Cassie and Sunny go in search of Hayley's family as the press begin to take an interest in the story. A detective who was involved in the original investigation offers his thoughts on why her disappearance remained unsolved, pointing out that Hayley's boyfriend was suspected of the crime but was never charged. The distance between where Hayley was last seen and where her body was found leads Cassie to form a new theory - that the killer may have travelled up from London, then returned before they could be questioned.

Cassie and Sunny begin their interviews...

It becomes clear to the detectives that the four men have conspired to come up with a false account of what took place the night Hayley disappeared. Jake is assigned to track down Tim Finch's ex-wife to hear her version of events - and uncovers disturbing information about Chris Lowe's past. On a personal front, Sunny is offered the opportunity to rekindle his past marriage, and Cassie worries about her father's failing memory.

James, Tim and Chris are exposed as having lied about their whereabouts on the night Hayley died. Pete reveals that he saw Eliot climbing into the holiday home through a window on New Year's Day, while leaves James struggling with the possibility that his son may have killed Hayley. Fran cracks a code in the dead girl's diary, suggesting that she met a local drug dealer on the night she disappeared.

Following the discovery of seemingly incriminating items found at his house, Tim insists he bought them for his girls at a fete in Middenham. However, the team are not buying Tim's story. A new discovery leaves the team shocked and devastated, hitting Cassie particularly hard.

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.