Welsh crime fiction has really found its voice over the last decade, both on the page and with shows like Hinterland, Keeping Faith and Hidden bringing the Welsh culture, terrain and language to the small screen. The quiet, sultry mood; the wild hillsides and valleys; the damp and decay; the pent-up frustration and anger – if it’s atmosphere you’re looking for, then Welsh crime shows have it in rusty old bucketloads. Available to stream via BBC iPlayer and S4C Clic from 9pm Sunday 13 October, Cleddau (AKA The One That Got Away) continues the tradition with a moody, grey murder investigation, plus a mean and restless love story mixed in.

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The show takes its title from the River Cleddau, which runs through Pembroke Dock, forming a salt marsh and estuary out towards Milford Haven. Sparsely populated, in places the ancient woodland rolls down to the riverbanks and it’s in the woods, overlooking the dramatic Cleddau Bridge that the violence occurs. DS Rick Sheldon (Richard Harrington) already knows Abbi Rayner has gone missing – his wife Helen (Rhian Blythe) works with her. And his fears are confirmed when her body is discovered with ligature marks around the neck. On a nearby tree, a heart knot configuration has been carved, bearing Abbi’s initials.

The killing resonates in the community. In 2011, two nurses were strangled in the same woods, seven days apart, with the same carvings on nearby trees. Instantly, Sheldon’s boss, DCI Alan Vaughan (Ioan Hefin), fears that this is the work of a copycat killer and calls in DI Ffion Lloyd (Elen Rhys) – a detective originally from Pembroke who now works in Cardiff for a major crimes unit. Back in 2011, she and Sheldon put a man called Paul Harvey behind bars for the Heart Knot Murders and he’s still in prison.

But is Abbi’s death a copycat killing? Before she disappeared, she was due to travel to Paris with her boyfriend Darrell, where they intended to get engaged. Darrell is a boxer and weightlifter whose fridge is full of steroids. Could he have killed her in a roid rage? Things are further complicated by Abbi’s ex-boyfriend Ryan, an artist, who has been texting her constantly. In fact, he lives right on the tract of woodland where she was found, and seems to have an obsessive personality. He has also disappeared, and his house is full of creepy sketches of tangled trees and drawings of Abbi.

Two bombshell revelations towards the end of episode one may well shift your ideas about who the killer might be, and their motive. Could there be a connection to the original case – and if so, what is it? No doubt more hidden secrets are to come, thanks to how director Cath Tregenna’s paces the story. Tregenna’s previous work includes Three Pines, Lewis and Law and Order UK.

So far, so police procedural, but the investigation is set to be a painful reunion for DS Rick Sheldon and DI Ffion Lloyd. One of the reasons Lloyd departed Pembroke was to leave behind the breakdown of her engagement to Sheldon. In fact, he cheated on her with his now wife, Helen. Both Sheldon and Helen are wary of Lloyd’s return. Initially, she is received frostily by Sheldon, even though her demeanour is all business. Soon, there are hints that her love for him still lingers and working together will be emotionally testing for both of them.

Cleddau is billed as a mystery and a love story, so be prepared for two kinds of tension. The clock is ticking for them to catch the killer before someone else is murdered, and Sheldon and Lloyd’s professional reputations are on the line. There is some wonderful acting here from both leads. Richard Harrington was the impassive, troubled lead detective in Hinterland, set in Aberystwyth. Here he plays a different kind of character – a family man who is gentle, friendly and easygoing. Will he be strong enough to avoid sinking into his past? Meanwhile, Elen Rhys’s character, DI Lloyd, carries the hurt of their breakup, compounded by the distance she has put between herself and her family. Now, her father is suffering from dementia.

Swathed in mist, loneliness and regret, we see West Wales in midwinter, drenched in rain, with long nights full of shadow. An ominous, dark ambient soundtrack augments the feeling. There are six, hour-long episodes to enjoy, with the dialogue in Welsh and English, with English subtitles on the iPlayer and Welsh subtitles on S4C Clic. If you enjoyed Hinterland or Hidden, Cleddau will be essential viewing.