Intense is very much the go-to word for Nordic Noir, the entire genre known for its deep, brooding storylines, but in Bordertown Season 3 (its final series), intensity is taken to a whole new level.

Those who have followed the exploits of Detective Inspector Kari Sorjonen in the previous two series, perhaps won’t be surprised by this intensity. Here is a detective of few words – his eccentric behaviours, lack of social skills, and unconventional thought processes not unlike Saga Norén in The Bridge. Like Norén, Sorjonen is something of an outsider, but unlike his Danish counterpart, Sorjonen’s family are as pivotal to his being as his police work.

It’s that family connection that provides the real emotional kick to this season, with Sorjonen’s wife Paulina, finally losing her battle against cancer. It’s a trauma that could either bring Sorjonen and his daughter Janina closer together or drive them apart.

Coming to terms with grief and life as a lone parent with a fast-maturing teenage daughter is a challenge enough, but for a man with an unconventional approach to communication and relationships, it’s a tightrope to negotiate, as slippery as the snow-covered roads in the starkly beautiful Finnish countryside.

Of course, life on the Finnish/Russian border is never straightforward, and Sorjonen soon finds himself tracking a serial killer who seems to have an in-depth knowledge of his past cases. Is Sorjonen himself the ultimate intended victim?

Like previous series, this central structure provides the backbone for five separate storylines, played out in one, two, or three-episode story arcs to make up the entire ten-part series. It’s a device that works well, keeping pace taut as individual cases are solved while allowing space for the wider storyline to develop.

It is that wider storyline that gives Bordertown its true strength. With 31 episodes across the three seasons – over 30 hours of drama – there’s real space to explore and engage with characters. Creators Miikko Oikkonen, Antti Pesonen and Matti Laine have created a piece not afraid to shock with brutality (there’s more than a fair share of uncompromising scenes and gore in series three) but aware that for an audience to engage, they need believable characters they can invest in over such a long story arc.

It is in that character work that Ville Virtanen excels as Sorjonen. Virtanen doesn’t just play the complex Finn, he inhabits him. A fascinating creation of tics, stumbles and physicality, this is almost as if watching a piece of choreography. Virtanen doesn’t go for caricature; this isn’t some parody of neurodiversity, this is a well-conceived, deeply convincing portrayal of a man whose thought processes may not be conventional but are no less valid.

It is in this final series that we discover the background to that unconventional thought process. Earlier series left Sorjonen’s behaviour as something of an unexplained mystery, but here we’re offered a glimpse into the detective’s childhood and a hypersensitivity that shapes the man decades later. Virtanen’s portrayal of a man facing his own traumatic childhood memories while investigating an equally traumatic case is a pure masterclass in emotion on screen.

Of course, every good detective needs his nemesis, and for Sorjonen it’s a return of previous foe Lasse Maasalo. Wonderfully portrayed by Sampo Sarkola, Maasalo and Sorjonen’s series-long skirmish is akin to watching two chess Grand Masters at work – both trying to remain one step ahead of the other.

Series directors Juuso Syrjä, Marko Mäkilaakso, and Jussi Hiltunen perfectly balance the tension throughout, moving from the large-scale police investigations into the inner workings of a solo mind. The true power is to watch the series as an epic ten-hour sweep (or indeed the whole saga of all three series), but direction is tight enough that even dipping into just one of the five crime cases feels like a satisfying adventure.

Cinematographer Mika Orasmaa also aids the atmosphere, capturing the Lappeenranta landscape hauntingly, utilising the winter shadows and low light as almost a character in itself.

After 30 hours in the company of Kari Sorjonen, his colleagues, family and adversaries, it’s something of a wrench to leave Southeast Finland. The future of the characters is unclear – lifestyle changes, new careers and new opportunities offer paths ahead – but for Bordertown it’s a fitting end to one of the strongest Finnish TV series of recent times.

Bordertown Series Three is available to stream as a box set on Channel 4 from October 24.