Shetland sets the scene for a very emotional Tosh exit following alarming development

When it was confirmed that Alison O'Donnell would be leaving Shetland in season 11, there was an outpouring of emotion from its loyal audience.

Having already reluctantly said goodbye to Douglas Henshall in the seventh instalment, it felt especially painful to lose O'Donnell, who is the joint longest-serving cast member alongside Sandy actor Steven Robertson.

After that news broke, questions also began to swirl about how much longer O'Donnell would stick around for and three seasons on, we've reached that point.

After telling Radio Times she would "hate to overstay" her welcome in November 2024, O'Donnell's exit was announced in June this year.

"Playing Tosh has been the greatest gift an actor could hope for," she said in a statement.

"I grew up in this job. It has shaped me as an actor, and as a collaborator. It's not going to be easy to leave it all behind, but I am excited for the future, for new adventures… to whatever may come next."

Details about the circumstances of Tosh's exit were obviously kept firmly under wraps. Was she, like O'Donnell, ready for pastures new? Or would her departure turn out to be something far more dramatic and sinister?

In the season 11 premiere, which aired tonight (12 November), we were given a hint of what could be in store.

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When the skeletal remains of an unknown male were found inside a car dredged from the sea, Tosh and Calder's investigation turned up two men who went missing back in 2017: local doctor Raymond Linden, the owner of the car, and Ukrainian contract worker Ivan Dudka, who had an emergency home appointment with Raymond just before he disappeared.

Ivan was staying at a hostel in Lerwick, but the address recorded in the surgery diary was in Isbister, about an hour away.

Too curious a lead to ignore, Tosh decided to drive there and began snooping around an underground tank. But she wasn't alone. There was someone else in there with her – and she suspected they could be hurt.

"Are you okay? Do you need help?" she shouted. But there was no answer, so she descended further into the bowels of the tank when the ladder she was climbing down gave way and she fell. Fortunately, Tosh wasn't hurt and managed to climb to her feet. But just as she regained her footing, a figure rushed towards the ladder while her back was turned and began to ascend.

"Hey!" she yelled while trying to get a hold of them. But they closed the latch behind them and fled, leaving a very concerned looking Tosh trapped inside.

Left with no other option, Tosh used her phone torch to look around, which is when she uncovered the remains of another body, just as Calder was leaving a message on her phone.

It had emerged that the skeleton found in Raymond’s car did in fact belong to him, while Ivan is very likely the man Tosh found in the tank.

But who killed them, and why, remains unknown – as does Tosh's fate.

There has been concern among some fans that the DI could end up leaving Shetland in a body bag. But while the situation she currently finds herself is fraught with risk, that would be an especially grim and deeply unsatisfying way for O'Donnell to sign off after her 13-year run.

While the writers aren't averse to challenging their characters – Tosh was raped in season 3, nearly blown up in season 7, and lost a friend after she was murdered in season 9 – killing her off wouldn't be fair to Tosh, a survivor who has earned the right to leave on her own terms, or the audience, who truly wouldn't want that for her.

But what this latest ordeal does suggest is that this could be the final straw for Tosh. We all have our limits and finding oneself trapped in a tanker – with its myriad risks, some of which are life-threatening – could force her to reevaluate her life, not only for herself, but for her husband, Donnie, and their young daughter, Louise.

If Tosh was to die, Donnie and Louise's lives would be upended in the most painful and tragic of ways – which also isn't something viewers want to see. There's also the numerous occasions Tosh has had to sacrifice precious moments with her partner and child, such as a recent trip to visit their extended family, because her job has had to come first.

Taking all of that into account, the reason for her exit is a no-brainer.

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There's also the issue of Billy, who was revealed to have committed a grave misstep earlier in his career last season and has since been demoted from sergeant to constable.

"He can't stand the sight of me," Tosh vented to Calder, reflecting on an awkward exchange with him following the discovery of Raymond's car.

"He blames me for the demotion," she added, visibly upset about the deterioration of their long-standing friendship.

But while Calder did acknowledge Billy had been "a bit off", she assured her that "he'll be alright, he'll come around".

Yet, even if Tosh and Billy can smooth things over, it's unlikely their relationship will go back to the way it once was. Too much has happened; regret, shame and a deep sadness has set in.

Though not likely to be the sole reason for Tosh's exit, it's yet another reason that seems likely to convince the DI that the time is right to say her goodbyes, in what is set to be an incredibly emotional farewell.

Shetland airs on BBC One on Thursdays at 9pm.

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