When Shetlander Thomas Malone has his murder conviction overturned after 23 years, which he's spent behind bars, Perez has to reopen an investigation into the 1994 death of local teenager Lizzie Kilmuir. Later, Perez gets a text from Cassie saying she is unexpectedly flying back to Shetland, so he and Duncan meet her at the airport.
Perez and Tosh are warned to stay away from Mathias Soderland by the Norwegian Secret Service, who tell them they're meddling in an operation they don't understand. With Soderland their prime suspect for Sally McColl's murder, Perez and Tosh won't be bullied. However, their investigation is suddenly derailed by a shocking discovery.
Following Jo's assault, the team is devastated. While Sandy feels guilty for not protecting her more, Jimmy tries to keep everyone focused, but pressure mounts on him to re-arrest Malone. However, in the absence of evidence, Perez resists. Adding to his stress, DI Cole and DS Jackson from the MIT arrive and begin picking holes in his investigation. Stunned that a search of Malone's farmhouse has not been carried out, Cole and Jackson conduct their own. However, when they arrive, Malone is nowhere to be seen.
Troubled by historical DNA evidence results, Jimmy has to face the fact that Duncan may have been involved in Lizzie Kilmuir's murder, and potentially Sally McColl's as well. A shocked Duncan is brought to the station for questioning, hurt that Perez would even think him capable of murder. However, the detective knows Duncan's lying - perhaps not about the murder, but about something else significant. So when Duncan finally admits he had an affair with Donna, it's not long before a bombshell is dropped.
Carla and Prentice Hayes are found murdered in a violent knife attack. Jamie Hayes is missing, initially leading the team to speculate that he could be responsible. But Jimmy is convinced that Jamie could not have mutilated his mother in this way. Aware of her altercation with Prentice the previous evening, Perez pays a visit to Olivia Lennox. She's in her room, shaken, with blood on her clothes. Could she be responsible or an innocent bystander?
Calum is rushed to the hospital after attempting suicide while in custody, which brings Sandy under internal investigation for providing him with a pen. Jimmy travels to Glasgow to investigate human trafficking, but has he brought his family into danger? Meaning, Zezi sees a way to escape from her captor.
Calder and Tosh discover the truth behind Rossi’s mission. They must identify Annie’s killer before he can exact his revenge.
Calder and Tosh turn their focus onto Lisa, until her confession unearths a shocking event and leads the team back to Annie’s inner circle.