At the manse, Alan suspects that Lisa is lying about her argument with Annie on the day of her murder. After a surprising altercation, he calls his sister back to the kirk, where Lisa makes a shock confession to Calder and Tosh – shining a light on a new murder suspect.

Meanwhile, an increasingly desperate Ian continues to struggle in the face of Noah's nightmares. To help, he makes the shocking decision to return to the scene of the crime…with heart-breaking consequences

Standing in Rossi's abandoned hotel room, Tosh and Calder realise that he's been conducting his own investigation into who killed Annie and Bergen. Their fears are verified when MI5 agent Rob Harding warns them that if Rossi is seeking justice for his former protégé, it will be fatal. As they put the pieces of the investigation together, the detectives uncover a failed MI5 operation in London which surprisingly leads them to a local family, as a chilling showdown unravels.

At the kirk, Alan organizes a memorial for Annie. However, the mood quickly sours when an unwanted face arrives to pay their respects, unsettling the family.

In the aftermath of the confrontation, Tosh and Calder interview Rossi, who gives them his reasoning and motivation for going after Annie's killer – a tale full of guilt and revenge.

In the aftermath of the tragedy, Calder questions Patrick about Annie who, while he didn't particularly like Annie or her relationship with John, insists he had nothing to do with her murder. Thanks to Angus' exaggerated testimony of his attack, Tosh and Calder finally get a search warrant for the facility. There, they find a hidden basement where sinister, possibly illegal, activity has taken place.

In an interview with Dr Mohan, she reveals the identity of the mastermind behind the operation. Tosh and Calder consider whether this person could have been responsible for the deaths of Annie and Bergen. However, before they can connect the lab to the double murder, they go in search of vulnerable teen Astrid Jakobson, who's suddenly gone missing. Believing her to be an important witness, Tosh and Calder are up against it, both worrying that Astrid's elaborate efforts to escape the Isles will put her in grave danger.

At the Bett house, Ian is stunned to find Rossi in the garden talking to Noah. He leaves with an ominous promise – Rossi will put things right for Annie. Hearing of the encounter, Calder worries what Euan Rossi is really doing in Shetland.

Rossi reveals details of Annie's hidden history, leaving Tosh and Calder to wonder whether they might be dealing with something bigger and more dangerous than previously thought. Noah gives a heart-breaking account of the fateful night but falls short of identifying the killer. He does, however, give Calder a strange, cryptic detail which spikes her interest.

As conspiracy theorist Angus Wallace monitors the marine research facility, the opportunity to infiltrate presents itself. When he fails to return, his friend Lisa raises her concerns to Rev Alan Calder.

Meanwhile, an emotional John Harris opens up to Tosh about his fondness for Annie. However, when a new connection to Bergen is unearthed, the team quickly grow suspicious of John's sons, Patrick and Fergus. As the Harrises find themselves under the police spotlight – and John wrestles the loss of his friend Annie - their family dynamic begins to unravel with shocking consequences

A picture of victim Anton Bergen begins to emerge, leading Calder and Tosh to question whether they're dealing with a revenge killing. Furthermore, strange details begin to suggest there's a hidden side to Annie Bett. They attempt to fill in the pieces by speaking to Bergen's employers Karin and Stefan Jakobson – a wealthy Swedish couple who arrived in Shetland six months ago - and discover Bergen was a nurse to their chronically ill daughter, Astrid.

Elsewhere, a hungover Ian is found slumped in an abandoned building, wearing bloodstained clothes. Unable to answer police questions, Ian is released. That is, until messages on Ian's phone shock Tosh to the core.

Meanwhile, at the mortuary, Cora reveals a shocking discovery from her post mortem.

DI Ruth Calder is talking to Malcolm Kidd, a jittery young man holding a shotgun. His brother's dead body lies on the floor between them. Tosh and the rest of the cavalry are waiting outside. It has to end at some point, but how?

Later that night, Tosh attends her friend's birthday party and witnesses the argument of two married friends, Annie and Ian Bett. Their nine-year-old son, Noah, tells Tosh his parents are separating. Before they leave, Annie arranges a meeting with Tosh the next morning – she wants to speak to her about something important. Annie and Noah are staying with John Harris, a mussel farmer, and his sons Patrick and Fergus. But when Tosh arrives the next morning, there's no sign of Annie's car, and she's not picking up her phone. Where are Annie and Noah?