When Thomas Malone has his murder conviction overturned after 23 years behind bars, DI Perez has to reopen the investigation into the 1994 death of local teenager Lizzie Kilmuir.

Perez is on the trail of an attacker, Tosh investigates an oil company in the hunt for Sally's killer, and DNA testing in the Kilmuir case yields a surprising result.

Jimmy questions Donna about the shocking DNA results, Alison arrives in Norway in search of Andreas Hagan, and an unexpected twist leads Perez and Tosh into the murky world of the Norwegian far right.

Ignoring Secret Service warnings, Perez and Tosh track down Mathias Soderland. Back on Shetland, a beach memorial service is held for Sally McColl.

Perez must re-assess the investigation in the aftermath of the attack. The MIT arrives on Shetland and sets its sights on Malone. Meanwhile, Tosh begins to have doubts about Lars.

Season Finale - 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.

When a prominent and controversial Shetlander is shot dead on their doorstep in broad daylight, Perez rushes back from his mother’s funeral to lead the investigation.

In the aftermath of the diving incident, the team searches for a link between the two bodies. Meanwhile, Perez must deal with the return of Donna Killick.

The hunt is on for Sandy's attacker, before a shocking confession causes tensions to boil over and some Islanders take justice into their own hands.

After the shocking events of the previous night, Perez must piece together what happened before a rampaging and vengeful Creggan finds his target.

Facing a dead end in the hunt for Galbraith’s killer, Tosh leads Perez to the remote island of Fetlar. Donna asks Duncan for one last favour.

As Perez closes in on the identity of Galbraith’s killer, he struggles with Duncan’s secret, while Donna has one more surprise in store.