At the heart of Penelope Skinner’s lively, frank and ambitious new play there’s a psychosexual dilemma of heart-rending plausibility: a pregnant young wife, an English teacher, finds her summer holiday sexual appetite thwarted by a right-on, green fanatic of a husband who’s lost interest.

Becky, played with a wonderfully transparent febrile intensity by Romola Garai, tries to get things moving by commandeering his stash of pornographic DVDs. But even her new baby doll silk nightie fails to rouse the inner beast, and her neediness is driving her slightly mad.

With hubby away on television commercial-making business in Amsterdam, she toys with porn-inspired fantasies involving a plumber (Phil Cornwell) who’s coming to fix the sweating, vibrating water pipes; and a local lothario, Oliver (Dominic Rowan), who delivers her second-hand bike in full highwayman fig — he’s rehearsing the village hall play, but also chimes with the latest porn scenario.

It’s a set-up that could go in many directions: a Ray Cooney farce, for instance, or a really dark tragedy with some grim, chaotic consequences. Skinner, another talented graduate of the Royal Court Young Writers Programme, with credits at the National (Greenland) and the Bush (Eigengrau), pulls back from any hard decision on this.

The result is a steep loss of intensity even as the situation gathers, with carefree bike-rides through the countryside (ingeniously staged by director Joe Hill-Gibbins on a platform with back projections and large fan as a wind machine) as prelude to violent sex games with Rowan’s cavalier and adulterous Oliver.

The scene where he arrives at the house to rape her at knife-point in a balaclava while she prepares her husband’s bed-time cocoa is followed by her own unlikely return visit to find Oliver’s wife (Sasha Waddell) calmly referring to the sex video she’s made and texted.

The writing doesn’t sustain the blackness, or traumatic implications of these later scenes, and the seduction of the widowed plumber seems theatrically lightweight, too. Skinner’s very good, though, at bringing in the “normal” village life to this fantasy diversion.

One agent of this is Alexander Gilbreath’s brilliantly played neighbour Jenny, who first arrives with bundles of baby toys for the new arrival then stays to confess her own buried unhappiness. It’s a neat link, too, that Jenny’s unseen Polish house-help doubles as the participant in Oliver and Becky’s threesome.

And there’s one further skeleton in the cupboard: a stash of Tesco plastic bags, yet more forbidden fruit as far as Becky’s superstore-hating husband (Nicholas Burns) is concerned. The poor dolt can’t help caring about the planet: “I see someone with a plastic bag it tears me up inside.” First things first, eh?