Under Salt Marsh review – Rafe Spall’s thrilling Welsh crime drama is clever, gripping TV

This carefully plotted tale of the investigation into a small boy’s death is a compelling, psychologically astute watch – which constantly pulls the rug out from under you

By ’eck – it’s grim out west. Such is the overriding impression wrought by Under Salt Marsh, a six-part crime drama set in the fictional Welsh town of Morfa Halen. As the title suggests, the town sits alongside the treacherously boggy lands, under lowering skies and just, but only just, above rising sea levels. The latter is threatening to make the defences the inhabitants are struggling to build obsolete. A huge storm is thought to be approaching and emergency evacuation warnings readied. Think of the place as a conservation area for the pathetic fallacy. There is a lot of actual and metaphorical gloom about. Much of it is attached to local primary school teacher Jackie Ellis (Kelly Reilly). Already bowed under the weight of her nine-year-old niece Nessa’s (Amara Atwal) disappearance three years ago, she finds the body of one of her pupils, Cefin, as she walks home across the marshes one night. The child has apparently drowned in a drainage ditch.

When detective Eric Bull (Rafe Spall), who was also involved in Nessa’s case, arrives to investigate the boy’s death, it becomes clear from his and Jackie’s immediate hostility that they have a fraught history – although its precise nature differs slightly from the one audiences have come to expect. It’s a series that is good at subverting expectations at every turn – not least in its delicate evocation of grief and the manifold ways a landscape can affect its people. Morfa Halen’s townsfolk are hardy and self-reliant, qualities born of their environment and isolation. But the drama poses the question of whether such independence serves a person or a community equally well under more extreme circumstances – be they meteorological or emotional – or whether a community can implode under the strain.

Creator Claire Oakley parcels out her plot carefully – possibly a little too slowly, unless your appetite for brooding shots of liminal flatlands and distant mountains is inexhaustible – and clues, questions and suspects accrue. A missing pair of blue wellies, a partial handprint in the mud and a saltwater/freshwater differential suggest the boy did not die in the ditch but was moved there from elsewhere. This in turn suggests another party’s involvement and the possibility that it could also shed light on Nessa’s fate.

The local landlady’s unstable son Osian (Julian Lewis Jones) has much to say about his belief that the town is being poisoned and an alibi for the night of Cefin’s death. A new lead comes in the form of the discovery, via a box of schoolwork and other mementoes that Jackie has kept, in which both Nessa and Cefin drew a picture of what looks like an astronaut before they went missing. But Cefin’s best friend James (Osian Emlyn) tells them it is of “the bee man”. This leads them to the people who “live in unison with the Earth” on nearby Spider Island. Kieran (Nigel James Bradley), their resident apiarist, was interviewed as a possible suspect in Nessa’s disappearance but Jackie’s knowledge of her home town and its inhabitants’ histories reveal a truth more painful to Cefin’s father than the arrest of his son’s killer would have been. Another great strength of Under Salt Marsh is its teasing out of the inevitable connections, crosscurrents and secrets that bind a community but can also fracture it under pressure.

We also get insight into the personal lives of the two main characters. Bull is unexpectedly recognised by one of the visiting men from the environmental institute, and we realise he too is keeping an important secret. The way the local people treat Jackie indicates some common knowledge of past troubles, and her casual assignations with James’s uncle Dylan (Harry Lawtey) – whose own alibi for the night of the probable murder is not fully established – may be becoming less casual than either of them is prepared for.

There were only two episodes available for review but, with a little leap of faith, viewers can surely look forward to a compelling, psychologically astute thriller that manages to weave a thoughtful meditation on the ramifications of isolation and suffering round its narrative poles. There is also the background presence of Jonathan Pryce as the dead boy’s grandfather, Solomon. So far, Solomon’s role has been limited to unleashing his sheep at the town hall meeting to show his contempt for the notion of evacuating the place, and later leading people in a shoreline hymn to commemorate his grandson. But you don’t hire Pryce just for this – Solomon surely has more of a part to play. We will have to wait, maybe slightly impatiently if the pace doesn’t pick up, and see.

• Under Salt Marsh aired on Sky Atlantic and is on Now