The Essex Serpent draws Claire Danes and Tom Hiddleston into its misty maws

► The Essex Serpent (six episodes) is on Apple TV+, weekly from 13 May

In unsettled times, a fictional monster is never just a monster. So Sarah Perry’s 2016 neo-Victorian bestselling novel, about a giant serpent menacing an Essex backwater in 1893, made much of the science vs superstition upheavals of that era of rapid change. But Clio Barnard’s confident and mistily atmospheric six-part TV adaptation has fashioned it into a classy period chiller that’s longer on watery terrors and forbidden love than intellectual jousting.

In the Blackwater estuary, an underwater monster (its swishing Jaws-style POV open only to us) menaces the village of Aldwinter, terrifying locals and drawing Cora, a wealthy and excitable fossil enthusiast, like a magnet. Barnard and scriptwriter Anna Symons keep things pleasingly ambiguous from the off: we’re as unsure as anyone if the beast is a real danger, a supernatural being or – as Darwin fan Cora (a vivid, quivering Claire Danes) longs to prove – an evolutionary throwback. With the discovery of the drowned, scratched corpse of local girl Grace, Barnard shows off a talent for ratcheting up tension with a stream of carefully equivocal scares: a fisherman near-drowned in a boat battered by an unseen force, children casting spells, a seagull causing havoc.

It’s a series that’s held together, though, by its central love affair, as the newly widowed Cora keeps bashing up against Tom Hiddleston’s very married local vicar. Their opposing ideas about whether the beast can be explained by science or religion strike sparks, right from their first Hardy-esque meet-cute, rescuing a stranded ewe from a bog. But this is an ensemble piece, bursting with the need to dramatise every late Victorian cultural development, so Cora’s companion Martha is a proselytising working-class radical, Cora’s friend Luke a surgeon transforming Victorian medicine, both more than half in love with her. As handled by Symons, the story keeps itself busy tracing the shifting currents of desire and friendship between all of them, set around the slowly developing monster plot. The results are sometimes dull but worthy (a storyline about social housing reeks of good intentions) but sometimes thrilling (Martha and Luke having a hilariously drunken hook-up, to get revenge on Cora).

Around this, Barnard’s fleet, questing camera roots the show in nature, the windswept marshes presented as both liberating and dangerous for Cora, traumatised by her years as a violently abused trophy wife. The dualities of a constricting city and rural freedom are underlined by the design of her London mansion – an art-filled Aesthetic-styled prison – contrasted with the clap board simplicity of village life. Barnard also fills each episode with unsettling but painterly landscapes, with wharf scenes framed like Whistler’s Thames lithographs, while the serpentine coast is swathed in green mists or topped with eerily streaky indigo ‘nightshine’ skies. A spare and faintly unearthly, cello-heavy soundtrack by Dustin O’Halloran and Herdís Stefánsdóttir cranks up the mournful feel.

As the deaths and disappearances mount up, and after Cora’s schoolroom talk about the heretical science behind dinosaurs prompts mass fainting, Barnard adroitly sours the rural idyll. She’s always been good with small-town tragedy and disapproval (The Arbour, 2010) and children’s cruelties (The Selfish Giant, 2013), and here she paints the villagers’ choking fear in telling, tightlipped exchanges.

Alongside this, the love story feels a tad underpowered, not Perry’s vivid creation of battling ideas and unsure attractions so much as a tasteful melodrama. As Hiddleston and Danes exchange yearning eye-meets, waltz with sizzling awkwardness at a party or share panting, cheek-to-cheek insights about their guilty attraction, their affair sometimes has the air of a wistfully Fabian Bridgerton.

Their strange bond is underlined by their country costumes, which (along with those of Clémence Poésy’s smocked and saintly Dickensian vicar’s wife) are oddly 20th century-inflected ‘rational dress’. Is it for Hiddleston’s legions of female fans that they sport stylish sweaters and coats fit for the Toast catalogue, while everyone else has period garb and battered hats?

Thankfully, not even curious clothing can dim the show’s excellent performances, though a gentle, understated Hiddleston risks being eclipsed by Frank Dillane’s energy as the self-destructive Luke. Danes’s crackling performance effortlessly captures Cora’s hunger for freedom and her clumsy wish to help others, her mobile face switching in a second from righteous, wide-eyed anger to an agonised grimace as superstitious Aldwinter turns on her ‘unnatural’ theories. She brings extraordinary conviction as well as a piercingly thin-skinned quality to Cora, a woman paying a terrifyingly high price to escape the gilded cage that oppressive Victorian values have constructed around her.