It never bodes well when a film you are watching is immediately reminiscent of a better piece of work. Unfortunately, that is the case with Threshold — a low-budget horror shot on iPhones by director duo Powell Robinson and Patrick Robert Young — which has a surprising resemblance to Alex Ross Perry’s jet-black 2011 comedy The Color Wheel.

Both movies feature an estranged brother and sister with messed-up lives who embark on a road trip together and rediscover a powerful sibling bond. Both of the sub 90-minute films were shot quickly and feature shocking endings. One could argue that the similarities are only surface deep — for example, Threshold is heavily improvised by its two leads while The Color Wheel is polished and rehearsed to within an inch of its life, but they are enough alike for it to be distracting, especially when one soars and the other, well, doesn’t.

Threshold opens with stressed brother Leo (Joey Millin) busting into the apartment of his sibling, Virginia (Madison West), who is seemingly in the midst of violent heroin withdrawal. Leo calls 911 but, seconds later, his sister is up and about as if nothing happened. Virginia  explains that she has been clean and sober for eight months, but the cult she joined to help cure the addiction has placed upon her a “curse,”  bonding her during some sort of satanic ceremony to an unknown man, so that she feels everything he feels, and vice versa.

According to Virginia, it’s this mysterious stranger who was actually “overdosing” in her apartment, and when, out of nowhere, she has an orgasm in a parking garage, it’s clear something very odd is going on. After a bit of mystical business involving coordinates being written onto Leo’s arm, Virginia, plus her thoroughly bemused and highly sceptical brother, embark upon a cross-country road trip to find the man, and hopefully break the curse (if that’s really what it is).

Threshold’s supernatural component often feels like little more than a plot device, as the horror Robinson and Young are more interested in exploring is the one drug dependency inflicts on an addict and their family. Despite once being very close, Virginia hasn’t seen her big brother in three years, and doesn’t know that he is married with a daughter, or that he has given up playing music in a band to become a teacher. They are practically strangers, who return again and again to memories of each other in a bid to glue their shattered relationship back together.

Millin and West have a certain chemistry, but their characters aren’t terribly interesting, a fatal flaw for a film in which, for the most part, only they appear. (In The Color Wheel, Carlen Altman’s JR and Perry’s Colin might be awful human beings, but they at least have the decency to be fascinating and funny with it). Worse still, Young and Robinson don’t seem to have enough material to fill Threshold’s already brief 78-minute running time, with scenes featuring Virginia and Leo singing “House of the Rising Sun” in a karaoke bar and messing around with a ouija board feeling too much like filler.

Keeping Threshold’s unearthly moments on the back burner imbues with real impact those scenes when Virginia is channelling the actions or emotions of the stranger. An argument that boils up seemingly out of nowhere results in a foul-mouthed rant delivered with impressive venom (“No wonder your cunt of a wife left you, you piece of shit!”). But what could have been an interesting take on possession ducks the issue a little too often, and Virginia and Leo prove weirdly incurious about the former’s condition. For example, Robinson and Young don’t provide a scene where Virginia tries to figure out how the curse actually works. Despite the fact this mysterious stranger has been able to send her rudimentary messages, she’s never tempted to try and make contact with him, or even explore ways to prevent him from dropping into her body unannounced.

At its heart, Threshold is about the connections that define us and the dangers that lurk when those bonds are damaged or break altogether. Virginia and Leo are both lost — her addiction torpedoed a promising law career, while he’s a failed musician going through a divorce and is stuck in a job he purports to hate. The film suggests that rekindling their relationship might just illuminate a way forward for them both, as could Leo’s bond with two-year-old daughter, Ally, the only positive thing that seems to have come from the wreckage of his marriage.

Despite going to some pretty dark places, there’s optimism here, too, the title suggesting these characters might be on the verge of important new chapters in their lives if only they could find a way out of their current malaise. What they are really searching for isn’t a mysterious stranger but a fresh start (and not just with each other).

Shot in 12 days by a three-person crew on iPhones (like Sean Baker’s Tangerine), Threshold wears its micro-budget indie credentials on its sleeve but is none the worse for that. In fact, it is visually impressive — one particular scene shot at an abandoned roadside church not only underlines the ramshackle state of Virginia and Leo’s lives but artfully frames the pair in doorways, so they look trapped and isolated. Some of the edits are a bit abrupt, almost like a camcorder being turned on or shut off, but that plays into the rough and ready, road-movie feel of the piece. Nick Chuba’s score is by turns propulsive and atmospheric, a perfect accompaniment to the dusty vistas and purple mountains that briefly appear as the siblings continue their journey.

For all its islands of promise, though, Threshold is really just a passable family addiction drama with horror elements that are never fully realized. Sadly, I suspect the only thing anyone will remember of the film in a few months’ time is its splendidly unhinged finale.

Andy Winter (@andywinter1) has worked in British magazines and newspapers for more than two decades. He has previously written about film for The Digital Fix and Film Inquiry. You’ll find more of his writing at the website andywinter.online.