The search for what happened to Hayley begins as the team travel down to Middenham to meet her family.

Cassie and Sunny begin interviewing the four male occupants of The Spinney over the millennium, who all have fairly consistent accounts of the time spent in Middenham. The rest of the Bishop Street Station team look at the timelines and leads from the original investigation.

Cassie and Sunny suspect that the men all colluded in lying about their version of events for that evening, but why? The press begin to circle as they find out that James has been questioned in connection to the Hayley Reid case.

Cassie and Sunny finally get closer to some truth about the night Hayley disappeared and the case takes a devastating turn for Cassie.

Under the glare of media spotlights, the team race to find the evidence to prove the identity of Hayley's killer.

Sunny is called to a dump sight in Haringey, where a headless body has been found amongst the freezers. Meanwhile, Cassie has been on medical leave and doesn't want to go back to do her final three months.

As Sunny and Cassie investigate they find that a graduating class of police officers could all be connecting to the case- some are still serving in the force.

Cassie and Sunny interview the four suspects, who all deny knowing the victim. Boulting discovers Walsh was cautioned three weeks before his death and believes it was with Ram, while Collier manages to locate the rest of Walsh's body.

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. The pathologist identifies a cause of death implying Walsh was murdered.

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. Unexpectedly, things take a devastating turn for Cassie.

Despite a tragic turn of events, Sunny and the team narrow down the suspects. Will they find conclusive proof of who the murder weapon belonged and succeed in bringing Walsh's killer to justice?