The farm is in lockdown and police search teams swarm over the property, as the Bains are rocked by Azir's revelation. Believing the identity of the killer lies in the reason why Ellen left Shetland, Tosh and Calder arrest Bobby Bain. An explosive interrogation does nothing to dampen Calder's suspicions.

Back at the farm, physical evidence of Ellen turns the spotlight on another member of her family, as the police continue to piece together her final movements.

As the Bains begin to unravel, Grace remains defiant, but her silence only serves to confirm the existence of a secret at the heart of this family. Piecing together all that they have learnt so far, Tosh and Calder move ever closer to an answer beyond what either of them could have imagined.

Tosh visits the Sadats to ask if Ellen's struggles might be related to their son's death. They maintain that Ellen barely knew Akmal but, after Tosh leaves, Farida begins to unravel as her husband Azir watches on, concerned.

A voicemail from the night before turns suspicion onto Bobby Bain, but Tosh resists Calder's demands to make an arrest. When Sandy finds evidence of drug dealing at Cal's house, a defensive James points the finger at Neil Bain.

Tosh is accused of harassing the Sadats and Procurator Fiscal Harry tells her to back off. However, the warning only deepens Tosh's suspicion. Combing through the case files, she discovers evidence of a surprising connection between Heather Bain and the Sadats' family tragedy.
Later, a surprise confession reveals the shocking next step in the final movements of Ellen Quinn.

In the interview room, the new prime suspect talks about his special connection to Ellen Quinn. He claims that his funeral rites were an act of love, part of an ancient religion. But Tosh and Calder see an attempt to destroy evidence.

While Grace berates Calder and Tosh over their handling of the case, Stella is stunned to discover Ellen had a secret friendship with Tom Knox and decides to pay him a visit.

Following Jean's shocking revelation, Calder sits on her fears about her father and Stella. However, when her secret background check on Amma returns a surprising link to Ellen Quinn, she must come clean to an unimpressed Tosh.

When secret letters signed off with the mysterious symbol are uncovered at the home of Tosh's prime suspect, the team ask if his pen pal might be an accomplice to murder.

At the same time, hospital security records bring one of Ellen's friends into the frame. Tosh and Calder track their latest suspect down to a remote location where they make a startling discovery.

Calder decides to stay on and investigate the previous night's discovery. Tosh welcomes her support as the revelation of a tattoo sends the case in a sinister new direction.

When a mysterious tribute is left at the crime scene, Tosh and Calder are drawn to the house of Tom Knox, a nervous and reclusive figure whose family have a longstanding feud with the Bains. Tom is guarded about his relationship with Ellen and the detectives grow suspicious.

Elsewhere, Sandy visits a mental health facility where Ellen stayed briefly before moving to London. He discovers that she was admitted by her cousin Heather and looked after by Dr Azir Sadat. Both blame Ellen's breakdown on her hard partying lifestyle but it becomes clear that there are hidden layers to Heather and Azir's relationship.

Tosh receives a shocking call from Cora at the mortuary with a horrifying development, while Calder's past and present collide when she hears a startling revelation about her father.

As Calder and Tosh race to prevent disaster, they are drawn to a horrifying fire on a remote hillside and a brand new prime suspect in this increasingly complex and unnerving case.

Calder and Tosh stand over the body of the latest victim, lying in a remote ruin at the water's edge. Tosh sees the hand of organised crime in the murder but Calder casts doubt over her theory.

On the other side of the island, crofter Agnes Moffat's morning drive is interrupted by a gun-toting Howell who forces her back home to look after the wounded Nowak while they wait for a means of escape.

Elsewhere, Tosh and Calder deliver the grim news to the victim's family. Grief and shock turn into action as they take matters into their own hands.
Calder is embarrassed to find her personal life becoming part of the investigation when Cal Innes, under Sandy's interrogation, uses her as his alibi. Unimpressed Procurator Fiscal, Harry Lamont, questions Calder's suitability for the case but Tosh comes to her defence.

Elsewhere, a family meal ends with an alarming discovery for Calder, while, at the mortuary, Tosh is faced with evidence which sends their case in a dark new direction.

After thirty years away, Met detective DI Ruth Calder returns to Shetland on the trail of Ellen Quinn – the scared witness to a London gangland murder. In Lerwick, Tosh pauses her investigation into a string of sinister animal deaths to help Calder find Ellen before it's too late.

Ellen's family are the influential and notorious Bains, led by dominant matriarch, Grace. Calder and Tosh discover her mother Stella and father Kieran haven't seen Ellen since she left for London six months ago and are surprised to hear that she's back in Shetland.

Matters are made worse when armed London gangsters - the experienced Howell and volatile Nowak - arrive on the Isles on a mission to silence Ellen.

On top of the twists and turns of the case, Calder is faced with figures from her past including her ex-boyfriend Cal Innes and estranged younger brother Alan – now the minister of their late father's old church. However, these siblings have starkly different views of their shared childhood.

As the case gathers momentum, Calder and Tosh are stunned by news of a brutal murder which has all the hallmarks of organised crime. However, there's another shock in store for the team.