A Met detective returns to Shetland on the trail of a vulnerable witness. DI Ruth Calder must team up with Tosh to find the girl before it’s too late.

Calder and Tosh must work together to find the armed killers and prevent further disaster. Howell and Nowak take refuge with a local and wait for a means of escape.

Unnerved by her discovery, Calder decides to stay on in Shetland. Tosh welcomes the support, as the revelation of a tattoo sends the case in a sinister new direction.

Tosh believes she has the murderer in custody and tasks the team with finding the evidence to charge. Calder suffers in silence with Jean’s revelation.

Calder initially reels from the awful news, but quickly becomes suspicious. Tosh worries Harry and Donnie with her pursuit of a controversial new theory.

As the investigation draws to a close, Tosh and Calder try to unravel the mystery at the heart of the Bains. The identity of the killer lies in the reason why Ellen left Shetland.