The Killing and The Bridge changed the face of the TV detective drama. The influence of Nordic noir now permeates the entire genre – even Midsomer Murders had a crossover episode set in Copenhagen. The Tunnel is its most direct descendent, an Anglo-French remake of The Bridge substituting the Øresund Bridge for the Channel Tunnel. But though the first season was a cover version of its Danish-Swedish ancestor, the second – and now third – series has headed off in another direction entirely.

In this new season, refugees huddle in the belly of rusty fishing vessel before masked men set the ship alight; it's the perfect starting point for this cross continental thriller. It's also a believable excuse for British detective Karl (Games of Thrones' Stephen Dillane) to reach out to his contacts in the French police. Soon he's partnered with Elise (Clémence Poésy) again for another warped case featuring missing kids, swarms of rats and other allusions to fairy-tale child snatcher the Pied Piper.

The Tunnel slightly lost its way in season two. But judging by these first two episodes of this third and final endeavour, it seems back on track with a gripping, complex investigation trawling the underbelly of human trafficking and those who profit from the refugee crisis. Finnish director Anders Engström (Wallander, Jordskott, Taboo) helps capture an authentic Nordic noir atmosphere, with every scene bathed in deep shadows, greys and muted tones.

Yet The Tunnel's real strength will always be the interplay between Dillane and Poésy. Their strengths and flaws perfectly complement each other. You could easily accuse the series of being overly dour, but there's a warmth and a spark of humour in their relationship built on mutual respect and understanding. They both draw on and pay tribute to their forebears – The Bridge's Martin (Kim Bodnia) and Saga (Sofia Helin) – while also adding their own layers. Poésy's Elsie is awkward but brilliant, less quirky than Saga but perhaps more realistic and equally fascinating. It's sad to see the end of this detective double act but at least the last series sends them out with a suitably engrossing and politically charged storyline.

The Tunnel: Vengeance will screen on Sky Atlantic, Thu 14 Dec, 9pm.