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.

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.