Starting with the dumping of a brutally disfigured, dying young man in the middle of the night, this series is set to focus on the terrorist threat posed by the ultra gauche – the extreme left. Jérôme Huguet, who played a supporting role in last year's Angèle et Tony, is introduced as activist Thomas Riffaut. Thomas is part of a tangled web of criminals and well-meaning volunteers, whose activities range from extreme violence to caring for destitute African families.
On their trail are police officers Laure, Gilou and Fromentin, although the events of the previous series, in particular Laure's killing of serial murderer Ronaldo Fuentes, are still having repercussions. How much that death with overshadow this series' proceedings remains to be seen.
We are teased with a few brief scenes featuring lawyers Pierre Clément and Josephine Karlsson (with more to come no doubt), while Judge Roban is yet to make an appearance, after the devastation he faced at the end of season 3.
The menacing, vindictive Judge Wagner is very much in evidence, as is a new police chief.
As always, my heart was racing through much of the episode, and there were some welcome flashes of dark humour as well as a truly stomach-churning moment that surely wasn't meant to be seen on the big screen (at least not by sensitive souls such as myself!).
Welcome back, Engrenages. We missed you!
Spiral (aka Engrenages) returns on BBC4 at 9pm on Saturday 9 February