Black Rock

"We are all dying. That's the truth. We have no idea what's gonna happen. We could be hit by a bus... but there are no buses on that island, so we can go out there and be safe from buses."

The speaker, Sarah (Kate Bosworth), wants to mend a bitter long-term rift between her two best friends Abby (Katie Aselton) and Lou (Lake Bell) by getting both to join her for a camping weekend on the small island where they used to spend their childhoods. To persuade them to join her, Sarah had told them that she was dying of an incurable cancer, and wanted "to have this last hurrah, just the three of us". Her friends quickly grasp that she is lying - so she strips down her tasteless deception to its underlying existential point: life's too short.

Once they are on Black Rock together, the three thirtysomethings are stripped down even further. Lou may be addicted to her mobile phone, but there's no reception out here. And on this island that "smells like childhood", the three head off together in search of a time capsule that they buried when they were ten, along the way still bickering about teen sexual betrayal as though it had happened only yesterday. "She started it!", Lou declares of her argument with Abby, her reversion to adolescent idioms complete.

Then they run into a party of three men, also hunting for innocence lost. Henry (Will Bouvier), Derek (Jay Poulson) and Alex (Anslem Richardson) are slightly younger than the girls. All six had attended the same elementary school together, and Lou had once gone out with Henry's older brother. Now the three boys have returned from a tour of duty in Afghanistan, their childhood dreams horrifically realised in traumatic experiences abroad and dishonourable discharge. As drunken Abby tries to prove to herself that she's "still got it" and can succeed as a sexual rival to Lou, things rapidly get very out of hand, and Sarah, Lou and Abby find themselves being pursued by armed professional killers out for revenge.

Playing a childhood game of hide and seek – only with life-and-death consequences – the women will soon become even more literally stripped down, forced to huddle together naked for warmth after their clothes get soaked through. "They're trained to kill people and they have guns," says a terrified Lou. "What do we have?" Yet from such extreme vulnerability, they will search even further back in time, beyond the formative years that they once shared, for survival instincts primordial and atavistic. Armed with spears that they fashion themselves, they will come back fighting.

With its cat-and-mouse games and terrified women in the woods, Black Rock plays out like a female-focused thriller-cum-horror – think The Descent on an island rather than in a cave – but it also (again like The Descent) has significant differences from your average conventional genre picture. For a start, there is its naturalistic, dialogue-driven focus on character, reflecting the earlier experience of both director Katie Aselton and screenwriter Mark Duplass in the 'mumblecore' movement. Then there is the characters' age – for these six are no longer the sort of nubile co-eds who normally frequent such films, even if they all seem in different ways to be trying to reclaim something from their youth.

"Please stop," one of them says in the climactic confrontation – but despite their visible exhaustion, both sides must play out to its bitter end a game whose stakes have become too high to allow for a simple forfeit or restart. It is violent and visceral, of course, but also a metaphor for the irreversibility of time, as these adults who were once children together see all the ambition and promise of their early years turned sour, and the American dream transformed by time (and Middle Eastern adventurism) into a nightmare.

At times the films' different elements and aspirations sit a little awkwardly together. Black Rock sets itself up in such a way that it seems important and entirely predictable that the friction between Abby and Lou should be resolved before the final credits. If this were a straight thriller, the women's actions could speak for themselves, as they work together once again for a common cause. Here, however, we also get a scene of reconciliation and a big, emoted apology, articulated at a point where really words no longer need to be said – and might easily be overheard. The whole sequence rather betrays the filmmakers' leanings towards human drama, exposing the occasional mismatch between these and more genre-bound concerns – but for the most part Black Rock is a tense, atmospheric and engaging affair, with an existential edge.