A farmer and his wife sit at their kitchen table, anxiously waiting for a knock at the door.  They've suffered, we learn much later, a tragedy in the past; and they live in a friendless and paranoid world, where your own guests check your identity papers and your personal failings are regarded as crimes.  But tonight the conversation focuses on more mundane concerns: bad weather, tight finances, failing crops.  Oh, and of course – the foxes.

Dawn King's prize-winning script is set in a vaguely post-apocalyptic England, where an unspecified catastrophe has choked the food supply and life expectancies stand at a few years.  According to the government, feral foxes are to blame – a claim we see supported by a disturbingly credible propaganda video, a potent mix of truth, exaggeration and out-and-out fantasy.  On the foundation of this concept, King builds a convincing and deeply uncomfortable portrayal of how a Cold-War-style totalitarian regime could work in the UK: one which feeds on fear, turns neighbour against neighbour, and maintains all the time a dogmatic focus on entirely the wrong enemy.

The power of King's script has already been recognised, but theatre company Master of None add an exceptionally strong performance, and a haunting visual style.  This is a dark production – the lights are literally kept low – a decision which proves the perfect match for the creeping, brooding menace of the script.  The austere set is dominated by the gable-end of a barn, eerily reminiscent of a gallows, and there's a clever simplicity to much of the direction.  A table doubles up as a bed; a simple step is enough to evoke the farm's punishingly rough ground.

Though all the performances are impressive, top honours surely go to Alexander Stutt as the Foxfinder of the title, a young and amiable functionary who finds himself wielding far too much power.  He brings a delightful, almost comic fastidiousness to the early scenes, and he also captures the callous facelessness of an all-powerful state: when he casually refers to the population as a "resource", you know he's never looked at the world in any other way.  But that very certainty is his greatest weakness, and later – as, cleverly, the farmer's wife begins to use it as a weapon against him – Stutt begins to embody a delicate and nuanced decline.  This is a man who's never faced doubt before, and when the seed is planted, its product consumes him.

Many plays end with a sudden twist.  In Foxfinder the turnaround is more gradual, but it's surprising all the same; the conclusion is morally ambiguous, and the final line is the perfect one.  Despite the 90-minute running time, I was never anything less than completely engaged, and I left the theatre with the sense I'd heard an urgent warning – a strident political demand which transcends the party spectrum.  Given the right conditions, the experience of Eastern Europe could be repeated here.  Foxfinder shows us how it could happen; now it's up to all of us to work out how to stop it.