9pm, BBC One

When Shetland’s pilot episode arrived in 2013, I was in no doubt that a series would ensue, because there was just too much that was too good, in satisfying primetime ways, for it not to happen. That it has become a strong export is no surprise: well performed, well structured stories, wrapped in stunningly bleak and brooding landscapes that never quite become too postcardy, it’s a Nordic-noir copy that has been successfully sold back to Scandinavia. But I also remember wondering just how many violent murder mysteries those small islands could plausibly sustain, before the storms of incredulity grew too strong, and the show came adrift and went floating off to bump against the implausible coastline of Midsomer Murders.

Ann Cleeves, whose novels the series is based on, seems to have had a similar concern. She has said that Wild Fire, the eighth book in her Shetland-based series about detective Jimmy Perez, will be the last. But, from the first, Shetland the TV show has set out to be its own thing (one of its greatest strengths, the character of Tosh McIntosh, Perez’s perpetually unimpressed, slightly wounded and always watchful sidekick, terrifically played by Alison O’Donnell, is entirely a creation of the lead writer, David Kane), and it has long since moved off from the books. The last time a story came adapted from Cleeves’s novels was back in the second series, in 2014. Still, as the show returns for the six-part story that makes up this fifth series, and yet another body – well, a bit of a body – gets washed up on that bleakly beautiful beach, you have to wonder: how long can it be before the mainland authorities, getting hit with yet another request for technical assistance to help in the investigation of yet another slaughter on the islands, begin to wonder just what the hell is going on out there? Maybe the entire Shetland series to date has actually been a precursor to another kind of show entirely: some sci-fi eco-horror, in which it is revealed there is some toxic nerve agent in the water, turning everyone into depressed psychopaths.

The roots that Cleeves planted remain, but as the series has grown, it has looked rather more to the inspiration of Wallander, the Swedish series that is Shetland’s true model and closest spiritual cousin. As with Wallander’s town Ystad, the tiny, troubled geographical spot Perez patrols has become a rupturing focal point where wider global ills suddenly come bursting to the surface. The last series saw the Shetland squad wondering about energy corporations and travelling to Norway to face the rise of far-right politics across Europe, while the body count mounted epically around them.

This time, the dismembered stranger on the shore sets in motion a case that seems to stretch across the continents to involve people trafficking, modern day slavery and western racism, not to mention the impact of fishing quotas on the local industry. Too much? I’m not sure. The new story is engrossing enough, although I suspect I prefer Shetland a little smaller. I’ll keep watching, in any case, if only to continue to appreciate the greatest example of an actor clicking with a character we’ve had on British TV for years.

If anything, as he makes the role more and more his own, Douglas Henshall is only getting better as DI Jimmy Perez, the decent, lonely detective watching the rest of us with quiet dismay and increasing hints anger. It’s the best stuff he’s done on screen. Even as the storylines grow more baroque, Henshall keeps everything in reserve.

There’s real strength about the performance, but, whether amused or in despair, he plays it softly, brilliantly natural. No fiddly bits.