Shetland

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.

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.

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.