A murder in a small Scottish town sends the community into shock, and local detective Annie Redford is thrust into her first murder case.

Following the discovery of a human heart on the shores of the loch, forensic psychologist Blake Albrighton is brought onto the team.

The police hold an appeal for the safe return of Jonjo. Anxiety is exacerbated by the fact that Jonjo suffers from bipolar disorder and has neglected his medication.

As Annie becomes more entrenched in the investigation, Alan feels increasingly marginalised and their relationship begins to suffer.

Despite being threatened with arrest by Quigley, Albrighton continues to obsess over evidence. About to leave, he hears something that makes things click into place. Unable to face Quigley, Albrighton risks his life to prove he's right.

Marr continues to be a person of interest and when a police search warrant is granted, his house and possessions are pulled apart, he finally snaps and goes after Evie.