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.

Perez questions Donna about the DNA results, Tosh 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 Norwegian secret service warnings to the contrary, Perez and Tosh track down Mathias Søderland, as the murder mystery continues.

Perez must reassess the investigation following Jo's attack, the MIT sets its sights on Malone, while Tosh grows skeptical of Lars.

Troubled by historical DNA evidence results, Perez has to face the fact that Duncan may have been involved in Lizzie Kilmuir's murder and potentially Sally McColl's as well.