Thomas Malone returns to Shetland after serving 23 years in jail for the murder of teenager Lizzie Kilmuir, a crime he swears he didn't commit. Whilst most of the islanders are convinced of Malone's guilt, the appeal and his subsequent release have thrown up the possibility that someone else could be responsible for Lizzie's murder.

The hunt is on for Malone's attacker, Malone seems to know who is responsible, but isn't keen to share the information. Perez and Rhona believe Andreas Hagan, the health and safety officer at Forst Energy is responsible for covering up corporate manslaughter and is their strongest lead in Sally McColl's murder. Meanwhile, DNA testing in the Kilmuir case yields a surprising result.

Perez questions Donna on the DNA result. Tosh arrives in Norway to track down Hagan, but a twist leads Perez and Tosh into the murky world of the 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 reassess 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.

The personal and the professional collide for Perez, as he finally closes in on the truth.