Shetland season 8 episode 4 opens with Peter Ayre, the prime suspect for Ellen Quinn's murder, being questioned by the police. But as Calder and Tosh are determined to find out why Peter was trying to destroy evidence in a murder investigation, Peter is adamant that his actions were part of an ancient religious ceremony and it was what 'Ellen would have wanted'.
Meanwhile, a forensics team are working their way through Peter's croft, photographing all sorts of strange things like animal skulls and weird things stored in his freezer. But it's not until the police uncover a box of letters that they really strike gold.
Over at the Bain farm, the family is fuming that the police managed to lose Ellen's body, and Calder and Tosh visit them to reassure them they have got Ellen's remains back where they belong. However, Ellen's mum, Stella, is more shocked to hear that her daughter was friends with Tom Knox, a family enemy.
Calder has some explaining to do when the background check on her sister-in-law, Amma, comes through and she is forced to admit to Tosh that she could be linked to Ellen's murder via some specialist cigarettes. Tosh is fuming that Calder didn't tell her this earlier and asks if there is anything more she needs to know... Calder says there isn't, but viewers know that she is hiding the fact her father potentially had a fling with Stella Quinn and that Ellen could be her half-sister.
Tosh goes to visit Amma to find out more, and she eventually discovers that not only did she speak to Ellen on the day she died, but they were both friends with Kirsty Bell, the woman that we saw Ellen with in London in episode one. Amma admits to introducing Ellen to Kirsty and that Ellen had called her to the holiday cottage the day she died, asking her to help her get rid of the stolen cash.
Amma is adamant that she had nothing to do with Ellen's death and wishes she had helped her.
After Cora confirms that Ellen died as a result of a cardiac arrest and manual strangulation, it is also revealed that sage-coloured polyester and wool fibers were found on Ellen's body. However, it's not much for Tosh and Calder to go on, especially when they discover that phone records show Peter's mobile was connected to his home wifi the entire evening that Ellen was murdered, with him also making many online purchases, meaning he now has an alibi.
The police decide that Peter must have an accomplice and so start going through the mystery box found at his home. It is full of handwritten letters from the one same person, who signs each one with the mystery symbol from Ellen's tattoo and the dead sheep.
It is then discovered that Liam Kenmuir, Rosemary's boyfriend, could be Peter's mysterious pen pal after it is revealed that his work pass for the hospital was used to enter the mortuary the night Ellen's body went missing. The police quiz him and he is adamant that he must have dropped it in town, but Tosh and Calder know he is lying and covering something.
Later, Cora catches Calder dropping something at the forensics department and while Calder covers what she is really doing, we know she is dropping her own hair in for a DNA test to see if she is a match with Ellen.
Tosh and Calder become more convinced than ever that Liam is Peter's sidekick when the police searching Peter's house find his hospital pass. The pair track Liam down at one of the holiday homes where they grill him on what they have found, but as he continues to lie, Rosemary walks in unannounced disheveled and holding a knife dripping with blood.
At the station, Tosh and Calder question Rosemary, who admits to killing sheep for fun and that she is the one who has been writing fan mail to Peter for years. It transpires that Liam has been covering for her and that she is jealous of Ellen and Peter's connection. However, despite all this Rosemary is adamant that she didn't kill Ellen.
After quizzing Rosemary, a call comes through for Calder while she is making a cup of tea and Tosh takes a message. It turns out that the DNA results are through and Ruth and Ellen are not sisters after all. But Tash is fuming that Calder kept this from her and asks her to leave the investigation with immediate effect.
Over at the Bain farm, Cal has gone to see Bobby and it looks like they might be about to go into business together with their dodgy drug dealing plan. However, the pair soon have a drink together and sample the drugs they are planning to sell - with Cal letting slip that there is a rumour going around that Stella had a fling with Calder's dad.
Bobby point blank refuses to believe it and asks Cal to leave, which he does after Bobby helps him fix his car when it won't start. As he drives off, Cal calls Calder but she cancels the call after an argument with Tosh and lets it go to voicemail.
Cal leaves a message telling Calder that he has just seen Bobby and that he is definitely hiding something... however, the next morning we see Cal's van has veered off the road and crashed, with Cal lying lifelessly next to the van - suggesting that Bobby has tampered with his car while pretending to fix it, possibly to silence Cal for talking about his sister's past.
Shetland season 8 is available on BBC iPlayer in the UK and BritBox, from November 29, in the US.
