We had been warned, before going into the screening, that Donkey Punch wasn't “a critic's film”. This rang alarm bells, but it needn't have. After nearly a week of disappointments, some crushing, most tolerable, this neat British horror-thriller stood head and shoulders above almost all of the fare on offer in Park City this year. Screening in the Sundance Film Festival's Midnight section, Donkey's Punch is a notable achievement in all departments, handled superbly by its first-time director and largely unknown cast, with a tight, frighteningly plausible script that carries the film through some of its more far-fetched moments.

It begins on an understated note, with three northern girls on holiday in Marbella, Spain. It's meant to be a no-guys event, but after a trip to a local bar they meet three cute English lads with a boat in the marina. They look harmless enough, so they accept an invitation to party, and before long they're out to sea, high on a mix of Ecstasy and Russian meth. The drugs heat things up, and during a fairly explicit five-way orgy, one of the boys gets a little over-excited and tries out the “donkey punch”, a sex tip they had been talking about earlier. It's a bad move: the intoxicated revellers soon sober up when it becomes clear that the girl on the receiving end is dead, killed by a broken neck.

So what do they do next? It's a conundrum that the film has a lot of fun with. The amount of drugs consumed on the boat implicates everyone, but for the boys the situation could look to a jury like a clear case of gang rape and murder, making honesty a risky policy. Despite fierce protests from the girls, they drive far out to sea with the intent of dumping the body and calling the emergency services a good 45 minutes later to register the victim missing. This might all seem a little extreme, but by this time the characters are vividly drawn: the girls are impulsive, reckless and easily scared, while the boys are largely middle-class, not-too-bright and have a lot to lose.

It takes a fair while to get to this point, so it seems - at first - that Olly Blackburn's film is simply going to be a moral maze, as the remaining six argue the finer points of perverting the cause of justice. Not so. Pretty soon, the film reveals itself as a very ambitious slasher movie, mixing the inventive gore of the Final Destination and Saw movies with the underlying humanity of Neil Marshall's excellent genre flick The Descent – a good reference point for Donkey Punch. Like The Descent, Donkey Punch has a respect for formula but, more importantly, a commitment to doing things differently, and properly.

Its sex, gore and some of its Ibiza-culture characters may not make it a critic's film, but it's definitely an audience's movie. In the UK alone, where Nick Love's broadsheet-slated geezer dramas have a lucrative working-class fanbase, Donkey Punch ought to make a lot of money. The twist in the tale is that this scary, truly edgy film deserves to.

Editor's choice

• The End: the worst movie endings ever
• Introducing The Times Proms Podcast
• Heath Ledger tipped for Oscar