Unrelated (15

This coolly intelligent British film from first-time director Joanna Hogg investigates a class of people that may get on your nerves, but its assured style and emotional restraint are still likely to get under your skin. The last film I can recall on the subject of Brits holidaying in Tuscany is Bertolucci's Stealing Beauty, which was enough to make me hope there wouldn't be another. One felt that the postcard sunsets, ravishing light, and Sienese countryside were being wasted on people so self-absorbed.

Like the Bertolucci film, Unrelated introduces an outsider into a group already well-settled into the holiday routine. But, instead of the coltish 19-year-old Liv Tyler, the newcomer is a fortysomething Englishwoman, Anna (Kathryn Worth), trailing a suitcase on wheels and a troubled relationship. The intermittent calls she makes to her (unseen) partner back in London come to feel about as relaxing as a wasp around a panini. She arrives at the rented Tuscan villa of her friends at night, late enough for her hosts to have gone to bed. Their teenage children are obliged to interrupt their poolside boozing and offer her a welcome.

Hogg's narrative approach is so unhurried one keeps wondering what she is holding back. The answer is not that much, at least in terms of action; what she offers instead is a richly nuanced psychological portrait of a woman at life's crossroads, hopeful of one more adventure and yet fearful that she has missed her moment. It turns out that Anna has rather drifted apart from her old schoolfriend Verena (Mary Roscoe) and her husband George (David Rintoul), but she does make a connection with "the kids", particularly the raffish, good-looking Oakley (Tom Hiddleston), an Eton boy with all the swagger of one born to it. A bond is formed across the generations. He knows Anna would rather drink wine with him than gargle on Negronis with his parents – "the olds", as he calls them – and she instinctively pals up with him on their day out at the Campo and among the humbug pillars of Siena cathedral.

Worth brings the right air of uncertainty to Anna, caught between the obligations to her contemporaries and the exhilarating mischief of the youth. Attractive without being a stunner, she's still in good shape and senses her own allure. At first it's a matter of sidelong glances. Then she goes skinny-dipping in the pool late at night, where Oakley and his brother Archie (Harry Kershaw) and their mate Charlie (Michael Hadley) gaze at her as if she were a sea-nymph rising from the waves.

Her first "grown-up" conversation with Oakley touches alarmingly on her own undisclosed issues back home: "Long-term relationships aren't easy – they're a challenge". A batsqueak of sexual possibility pierces the air, only to be politely but summarily dismissed. It's the entirely casual nature of the brush-off that feels so accurate. Anna, waiting for a sign, has perhaps misread the whole situation, and you can almost feel the chill in her bones as she realises how thoroughly she has been kidded – or been kidding herself.

The Rohmer-like atmosphere of romantic fancy dissolves very suddenly, and the film, metaphorically speaking, goes from summer to autumn in a moment.

Once again Anna has to reassess her position as interloper, and the choice she makes in exacting a, perhaps unconscious, revenge on Oakley backfires badly. A minor car accident, kept secret from the parents, is brought to light, and Oakley's involvement in it causes a tumultuous shouting match between him and his father, not seen, but heard at full volume, by the rest of them as they lounge by the pool.

The family idyll seems to fray, and Anna finds herself marginalised by agonising degrees. The film illustrates this in subtle compositions, with Anna lagging behind on walks, ignored like a wallflower at a locals' lunch-party, and then quite invisible to the children when she's dispatched to fetch them from an upstairs room – at the top of a staircase she sees them all with their backs turned, gazing out of windows as at a future that specifically excludes her.

What makes her plight somehow more poignant is the sultry Italian atmosphere enveloping her. Oliver Curtis's photography renders the blue-black nightscapes, bloodshot horizons and glamorously shaded interiors a place which Anna at last finds too beautiful to bear; she flees to an anonymous chain hotel where she can nurse her misery undistracted. The manner in which the screenplay eventually discloses her emotional distress is probably the weakest passage of the whole film: something previously guessed at, in silences and gestures and even just faces caught in profile, becomes something too literally explained. It's not that we don't believe her, merely that those subtle gradations and flickers of feeling held a more lifelike ambiguity.

Unrelated is an accomplishment, all the same. With its overlapping conversations and contemplative moods, it feels significantly different from the British mainstream, clogged with romantic comedies, mockney gangster flicks and period adaptations. It is not only its setting that aligns it with European cinema; it has to do with the luminous sense of space and the stillness of the camera.

If Hogg can render the travails of a bunch of middle-class British holidaymakers a subject of interest, there's reason to hope she has some career in the making.