Joe, a rootless young drifter, finds work on a barge travelling between Glasgow and Edinburgh, owned by Les and his wife Ella. One afternoon they discover the corpse of a young woman floating in the water. Accident? Suicide? Murder? As the police investigate and suspect is arrested, we discover that Joe knows more than he is letting on. Gradually we learn of Joe's past relationship with the dead woman. Meanwhile an unspoken attraction develops between Joe and Ella, heightening the claustrophobic tensions in the confined space of the barge.

David Mackenzie is the young British director whose debut feature, The Last Great Wilderness, was an intriguing, untidy beast of a film: a road movie which wound its way up into the Scottish highlands to uncover a sort of Wicker Man serio-comedy. I liked it; others didn't. His new film, a reverent screen revival of Young Adam, the novel of sinister and transient bohemianism by the all-but-forgotten Scottish beat author Alexander Trocchi, is a conspicuously more mature piece of work.

Everything about this adaptation shows it to be a labour of love: the intensely focused performances, the lugubrious and sensuous cinematography by Giles Nuttgens and intelligent production design from Laurence Dorman that conveys 1950s Glasgow without excessive expense. It's a dreamy, disquieting study of sexual tension and guilty secrets. The movie drifts downriver, like the tatty barge on which it's set, towards its finale at a sensational murder trial, resembling something out of Witness for the Prosecution.

It has its faults - implausibility and absurdity in its sexual imbroglios and a narrative structure that tends towards the elusive. But this is really impressive, accomplished work from Mackenzie, who is showing himself to be a natural filmmaker.

Young Adam is about a crime of passion, which is neither exactly criminal nor exactly passionate. It all revolves around the corpse of a young woman dragged out of the river Clyde one day by two itinerant barge workers. They are Joe and Les, played by Ewan McGregor and Peter Mullan; Joe is a shiftless sort of guy, always lethargically puffing at a roll-up, having an unearned sit-down and gazing broodingly into the middle-distance. Les is an older man who is married to the barge's owner, Ella (Tilda Swinton). She is technically the employer of both men, and lives cheek-by-jowl with them and her little boy on the narrowboat, hauling coal - dirty, cramped and exhausting work which leaves them black with dust. The men become like two brothers, matter-of-factly scrubbing each other's back and complaining to Ella, as if to their mother, about who gets the hot water and eggs for tea. But after a punishing night on the drink, Les can't satisfy Ella in bed and inevitably she begins a hungrily physical affair with Joe - an adultery which is played out with flashback scenes of Joe's mysterious past affair with Cathie, played by Emily Mortimer.

For my money, this is the best performance of Ewan McGregor's career by a long way: subtle and complex. He's no straightforward good guy; his quiet, personable demeanour conceals weakness and arrogance - a would-be writer who abandoned his vocation and now resembles an artist only in moodiness. When asked by Les how he thinks the dead girl died, Joe riffs prose-poetically about how he imagines her undressing, her clothes fluttering "like pennants in the wind" before throwing herself into the water. Ella and Joe do not remark on this departure from his usual laconic monosyllables. But we are to find out just how culpable Joe's flight of literary fancy is.

Peter Mullan gives another excellent performance: a humane and compassionate view of Les, the hard drinker and hard worker who sees nothing accumulating in his life. Tilda Swinton is always a riveting screen presence - a natural movie star - even when her commanding and patrician charisma cannot quite be accommodated in the confines of a particular role, which I think is arguably the case here. Her Ella is dowdy and fierce, keen to keep her menfolk up to snuff; she's angry and confused about the unruly sensuality awoken in her, and then blooms into beauty as she cultivates a long-dormant taste for pleasure in bed with Joe. All this happens in tandem with Joe's past romance with Cathie, which confusingly overlaps with the present as he begins his job on the barge while their love affair acrimoniously crumbles. Mackenzie gets a very strong performance from Emily Mortimer, too - and Mortimer has to carry the film's most bizarre scene.

At the climax of a blistering row, Joe tears Cathie's clothes off, empties custard and ketchup all over her, takes what looks like a riding crop to her backside and finally forces himself on her. Is it a rape scene? Well, the mandatory 21st-century repudiation of political correctness might appear to rule out such a straightforward reading. There is, moreover, an ambiguous aftermath. Joe storms out to the pub and later returns to find that Cathie has submissively cleaned up the mess and is forgivingly waiting for him - in bed. The act appears to exist within the mysterious unknowability of what is effectively a bad marriage. But what we are to find out later about Joe's weakness and self-centredness certainly shows his readiness to betray and defile Cathie in a more intimate way still.

The closing act shows Mackenzie slightly at a loss, I think, about where to take Joe. His sexual adventures verge on the ridiculous - is there any married woman who isn't going to get nailed? - and Ella is remarkably trusting about letting him go to the pictures with her widowed sister. But the movie is wound up with a robustly dramatic and vividly demonstrative trial scene which endows it with solidity and permanence.

Mackenzie has a sure, visual touch and a mastery of cinematic language, at least in embryo. His movie resembles Lynne Ramsay and Terence Davies; it's a little like Richard Jobson's Sixteen Years of Alcohol and there are echoes of David Cronenberg's Spider, based on the Patrick McGrath novel, and Mackenzie is in fact now working on a version of McGrath's Asylum. He is a British filmmaker to be proud of - and excited about.

EWAN McGREGOR Joe Taylor
TILDA SWINTON Ella Gault
PETER MULLEN Les Gault
EMILY MORTIMER Cathie Dimly
JACK McELHONE Jim Gault