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.

A severed hand washes up on a Shetland beach, with further body parts discovered in a holdall that has been dumped at sea. Perez and the team begin a complex murder investigation.

Perez finds that the murder of the young Nigerian man is more than just the drugs trade but could be sex trafficking with the Shetlands a cog in a much larger wheel.

Hoping to find information on Paul Kieran, Jimmy goes to Glasgow. There, he and DI Sam Boyd from the trafficking unit pursue some leads. Plus, Jimmy can't visit Glasgow without seeing Cassie, which brings her to the attention of the people tailing Jimmy and puts her in danger, too.

Having heard of the traumatic events in Glasgow, an angry Duncan confronts Jimmy over his fears for Cassie's safety. Cassie, although shaken, seems pleased that Jimmy has trusted her with the truth. Grant, young plain clothes officer, is tasked with keeping an eye on her. Now sure that Zezi is still being held on Shetland, Jimmy and Sandy pay a visit to Morag Dunwoody in an attempt to get more information from her. Her attitude has changed completely from the day before, when she admitted to Sandy that Calum didn't transport the girls to the mainland. Morag is obviously shaken and Jimmy suspects that she's been warned off by the traffickers. That night, Morag's home is set on fire, but she escapes, terrified, with her young children and later admits that she received a phone call before the attack, from a woman with a Glaswegian accent. Andrea Doyle, the tattooist, becomes a potential suspect. Very affected by his kiss with Alice, Jimmy tries to meet up with her, but the investigation keeps getting in the way. When he finally does get the chance to see her, Alice admits that she's not sure how she feels about the situation and whether she still loves Chris or not. While Duncan is out for a drink, he sees Jimmy and Alice chatting intimately in a corner - but so does Chris. Sandy pulls an all-nighter going through CCTV and tracks down Andrea Doyle at a gas station near the Dunwoody's house prior to the attack. They manage to find an address for her and she is arrested after a chase. Frustrated by not getting any closer to finding Zezi and convinced that Jamie Hayes knows more than he's letting on, Olivia ambushes Jamie and Rosie, and drives off with Jamie in the car.

The discovery of Nikki's body intensifies the search for Zezi. Perez feels that time is running out. A desperate Olivia tries to get information from Jamie Hayes.

Perez closes in on Zezi's location at last. But can he reach her before it's too late?

Alex Galbraith is shot dead on Pérez's mother's funeral. Their investigation is interfered by Fraser Creggan. A pipeline worker Eamon Gauldie informs Pérez that he has evidence on his drone that can identify Galbraith's killer.

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

The race is on to catch Sandy's attacker before he can escape Shetland, while Lynda Morton finds herself in trouble, so she desperately calls Perez for help - offering a key name for his investigation in return. Blood from the recent break-in throws up a DNA match that leads to another murder suspect, and a shocking confession causes tensions to boil over, sowing the seeds of further violence.

After the shocking events of the previous night, Perez must piece together what happened before a rampaging and vengeful Creggan finds his target. Eve denies any involvement and has an alibi, while Tosh visits Maurice Ross to find out why Galbraith was paying for his care. At the religious retreat on Whalsay, Sister Carolyn receives a threatening postcard and quickly packs her bags.

Perez finds that Carrie McAndrew was in debt to Niven Guthrie. Fiona Bedford reveals that her husband is tracking her whereabouts. As Donna grows weaker, she asks Duncan for one last favor.

Niven and Darren are arrested and brought to the station for interview. Perez finally brings the investigation to a climax - but he has no time to rest when it turns out Donna has one last surprise in store.

DI Perez investigates the disappearance of a vulnerable young man, whose family are new to Shetland.

After the discovery of an unknown body, the search for Connor grows increasingly desperate. Perez follows Danny on a perilous journey into his murky past.

Mounting evidence pushes Perez and the team to revise their view of Connor Cairns. Is he a vulnerable missing person or a killer on the run?

Perez issues a warrant for Connor's arrest and looks for the motive behind his bomb-making. Sandy tries to find out what business William Rodgers had in Shetland.

Perez tries to get the measure of Lloyd. Evidence emerges of another bombmaker. The net closes in around Martin. Sandy uncovers Connor's interest in a piece of local history.

The team races to identify the killer and prevent disaster striking Lerwick. As the investigation draws to a close, Perez faces a life-changing decision.