Calder has to deal with some devastating news in Shetland

As she’s been quick to point out during the series so far, Tosh may only be an ‘acting’ DI, but in reality, she’s far more than that.

Ever since Shetland began, the character, played so beautifully by Alison O’Donnell, has been the crime drama’s beating heart, a person we’ve seen mature and develop before our very eyes. Douglas Henshall’s Jimmy Perez may have been the principal figure, but it was Tosh we really adored and willed to do well.

So there’s probably a sense of satisfaction in seeing her blossom without her mentor, stepping up into his well-worn shoes to take charge of proceedings at Lerwick police station.

“Tosh is very comfortable in the role and has really found her rhythm,” says O’Donnell. “She has an easy rapport with Sandy (Steven Robertson) and Billy (Lewis Howden) and the team seems to be functioning well. She also has a great connection with Harry (Tibu Fortes), the new procurator fiscal, who really champions her.”

Although she has the backing of those around her, Tosh’s latest case has proved to be a tricky one, not helped by a love-hate relationship with DI Ruth Calder. The latter was born and brought up on the islands, but left under a cloud many years ago; she’s only returned to find the evidence needed to sort out a case on her beat in London.

“The relationship between Calder and Tosh is restless,” claims O’Donnell. “It is new and fragile and their policing styles don’t always marry up. But these are two confident, experienced women who recognise the value of working together and pooling resources, and who respect each other, even though they don’t always see eye to eye.

“There are moments when Calder must accept that her tough, urban approach isn’t the best way to get results from a small island community and there are moments when Tosh must step aside and let Calder lead with all of her expertise in organised crime. There are moments of fracture, of doubt, and there are times when they really click and are absolutely on the same page.”

Thankfully, O’Donnell’s relationship with her new co-star, Ashley Jensen, who plays Calder, is far more easygoing: “I had an absolute blast working with Ashley. Her approach to the job was pitch perfect. She was really open and collaborative and sensitive to the fact that this is a long-running show but at the same time she brought confidence and self-assuredness, both in terms of how cast-iron her characterisation of Ruth Calder was, and in terms of how she took the reins off-set and brought everybody together.”

The two characters could use some of that understanding as we head into the penultimate episode.

Calder has to deal with some devastating news, as well as a disappointment – Tosh refuses her demand to arrest Bobby Bain, despite receiving a revealing voicemail about him.

Tosh is also warned off speaking to the Sadats about their son, his death and a possible link to Ellen’s problems, something that only makes her more determined than ever to delve into the story – and leads her to a shocking revelation.