Forest-bathers, wild swimmers and other back-to-nature evangelists get a hearty rebuff in Joe Lo Truglio’s debut feature, a sturdy psychological drama with a horror chaser in which the immersion in the great outdoors is the problem, not the solution. Traumatised cafe owner Kate (Beth Dover) decides that the cure to her shell-shocked spirit is a three-month spell looking after a fire-spotting tower in the wilderness, but the swarm of flies infesting the balcony is an early warning that nature can just as well deliver trouble as solace.

With flashbacks to maggot-ridden cadavers and physical assaults, Kate is a double victim of abuse: childhood sexual assault by her uncle and, recently, domestic attacks by an ex-partner. Landing the volunteer gig in the mountains via her friend Nickie (Ta’Rae Campbell), she has to make twice-daily weather reports to Nickie’s brother, Earl (Ato Essandoh). But this hermit posting is as much a test for Kate as it is an escape: can she live up to the responsibility? Prone to panic attacks and hallucinations of being pinned to store counters by the townsfolk, she often has to retreat to her outhouse to hyperventilate things out.

Out on a ridge, the observation tower serves as an effective symbol for an isolated mind clinging to a fixed purpose. Slotting Kate’s psychotic breaks matter-of-factly into her daily routine, and investing in her stop-start friendship with her neighbour Reggie (veteran character actor Dylan Baker), Lo Truglio builds an engrossingly claustrophobic sense of an embattled psyche. In fact, he pulls something of a mean trick: setting up his film under the guise of a well-meaning drama about human resilience, before swerving into something totally nihilistic.

Initially performed with a slightly incongruous general chirpiness, the film then blazes over the top into a cartoonish frenzy. But otherwise it’s a well-conceived disintegration, with clear sight of the terrain, both outer and inner.

• Outpost is available on digital platforms on 11 September.