Ruth had arrived on the isles to track down a key witness in a London gangland shooting, Ellen Quinn, but their search took a tragic turn when Ellen's body was discovered.
It was a local who had murdered her and not gangsters Howell and Lukas, as it was originally suspected, but despite that, Ruth decided to remain in Shetland to assist with the inquiries.
After assuring everyone, particularly herself, that she would leave her hometown at the first opportunity, the detective wanted to see this case out.
But that might no longer be possible following tonight's episode (Wednesday 22nd November), in which Ruth's fraught family history clouded her professional judgment.
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Suspicious of her sister-in-law Amma after linking her to Ellen via a cigarette, Ruth did some digging and learned that Ellen had been introduced to her old flatmate Kirsty through Amma, and that the two women had spoken to one another on the day of Ellen's death.
She had reached out to Amma for help, but she wanted to keep her nose clean and told Ellen to go to the police. That, as we now know, didn't happen - and a few hours later, she had been killed.
But that wasn't the only development that Ruth had kept under wraps.
She also used her team in London to run a DNA comparison between herself and Ellen after learning that her father had been in a secret relationship with Ellen's mum Stella - and, understandably, Tosh was furious.
"DI Calder, I want to thank you for your week over this past week, but we no longer require your assistance on this investigation," she said. "It's time for you to go home."
Where does Ruth go from here?
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And there were two other seismic developments in the latest episode.
Ellen's best friend Rosemary was revealed as the sheep killer. She had been in contact with Peter Ayre for years, and he had used her boyfriend's hospital pass to steal Ellen's body from the morgue. But she firmly denied murdering Ellen.
And in another twist, Ruth's ex-boyfriend Cal might be dead after his van swerved off the road while he was driving back from a drinking and drug-taking session with Bobby Bain.
His night ended abruptly when Bobby chucked him out for asking questions about his sister Stella and Ruth's father.
There's a strong possibility that Cal crashed due to being under the influence, but there's also a possibility that Bobby cut his brakes when he was taking a look at his van.
Is Cal really dead? And crucially, who killed Ellen?
Shetland season 8 continues on Wednesday 29th November at 9pm on BBC One and iPlayer. Ann Cleeves's Shetland novels are available to buy now. Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to find out what's on.