Police, Adjective, Corneliu Porumboiu, 113 mins (12A)

It so happens that this is a great week for claustrophobic minimalism.

You've got the nerve-racking thriller Buried (see Nicholas Barber's review, above) about a man trapped in a coffin. And then you've got Police, Adjective – an anti-thriller that could well make you wonder whether confinement in a coffin might not be more joyous than loitering on the streets of small-town Romania. It's true, Corneliu Porumboiu's film contains long spells of what critics euphemistically call "dead time" – ie stretches in which nothing seems to happen, and very slowly at that. But if ever a film was worth sticking with, Police, Adjective is it, the slow accretion of everyday drabness building to an astonishing dramatic pay-off. Porumboiu made his name with the satire 12:08 East of Bucharest, and you'll find some humour here too, albeit as laconically downbeat as humour ever comes.

The film begins on drab city streets where a young man is being tailed by a hunched, unshaven individual in a cheap windcheater. The pursuer is a young policeman, Cristi (Dragos Bucur), usually seen skulking behind lampposts or hiding under the raised collar of his jumper – he might as well be carrying a neon sign reading "UNDERCOVER OPERATION – DO NOT DISTURB". He must be on a really important case to devote this much time and effort to it: but no, Cristi's targets are minor offenders, three youths who meet in a playground to exchange an occasional joint.

It's clear that Cristi's heart isn't in the job: when his painstakingly handwritten reports scroll down the screen, they read, "The suspect met no one, didn't use his phone and smoked one cigarette"; and, encapsulating the film's sardonic comedy, the killer phrase, "nothing happened for two hours".

In fact, Cristi is not merely bored by his work but repelled by it. Why bust these kids, he reasons, when Romania's law on cannabis is bound to change in a few years? He doesn't want to get a result by having one brother snitch on another – it would be on his conscience to give someone else a bad conscience.

Between episodes of Cristi's stakeout, we see this saddest of sad sacks at work and at home. At odds with the usual cop-shop bustle seen in even the cheapest of realist police dramas, the station here is a hospital-like corridor, with Cristi sharing a cubicle with a sullen colleague. There's some neat office-politics business, notably a moment with a co-worker who wants to lose weight by joining the police tennis club. The only meagre power Cristi gets to wield is in giving this man a curt brush-off.

Meanwhile, Cristi suffers embarrassment at home from his better-educated wife (Irina Saulescu), seen enjoying a schmaltzy ballad (over and over) on YouTube. When Cristi questions the dopey lyrics, she firmly puts him right about symbols, metaphors and pronominal adjectives. "Who decides stuff like that about words?" he asks, baffled. "The Romanian Academy," she replies.

But there are more baleful forces at work in Romania determining the use of language. In the film's culminating sequence, Cristi and his colleague are summoned by their boss Captain Anghelache, who wants to know why the stakeout hasn't produced any results. Cristi states his moral qualms – but Anghelache calls for a dictionary and has Cristi read out the definitions of the words "conscience" and "morals". It's an astonishing scene, the tension building drip by drip as the Captain humiliates both men – Cristi's office-mate being made to write on the blackboard like the gauchest of disgraced schoolboys.

What makes the scene so perfect is the casting of Vlad Ivanov as the Captain: he was the abortionist in the extraordinary 4 Months, 3 Weeks, 2 Days. A bland, baby-faced individual, soft-spoken to a fault, Ivanov never raises his voice, indeed barely stirs in this excruciatingly extended sequence, yet he terrifyingly embodies bureaucratic power at its most coldly unbending. Shot in two long takes, this scrupulously undemonstrative scene lays bare the essential matter of power, politics and morality, as the Captain uses the spurious logic of language – "Lads, do you know what we're doing here? Dialectics" – as a merciless weapon.

The last definition that the hapless Cristi has to read out is of the word "police" – and it's here that writer-director Porumboiu makes his point both about oppressive regimes and about the artificial excitements of the police-procedural thriller. Police, Adjective makes the gritty-realist longueurs of The Wire look like Transformers, and the mundanity of The Bill look like Miami Vice. But Porumboiu's film has a brittle sensibility and a philosophical bite that are very much its own – discreet, very Romanian and, in a downbeat way, altogether devastating.

Next Week:

Jonathan Romney watches Mr Nice, about the life and high times of counter-culture hero Howard Marks