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Oculus should have been a short film. It’s competently acted, relies heavily on atmospherics, takes place almost entirely within a small set, and director Mike Flanagan – who also co-wrote the script and edited the film – shows a subtle confidence with his camera. He doesn’t overplay the gore factor, and when demonic spectral forms do start showing up, they’re used more for psychological oomph than for cheap scares.

Anyway, those are the positive aspects Oculus shares with short films. The negative aspect is that it doesn’t really have anything to say. At 15 minutes, a lack of real thematic substance is forgivable. Not at 105 minutes.

It’s a real shame, because things start strong. Tim Russell (Brenton Thwaites) has just been released from a mental hospital. Years ago, his father brutally murdered his mother before being shot by Tim. That got the boy institutionalized, and his slightly older sister Kaylie (Karen Gillan) shipped off to foster care. Once Tim is out, the siblings reunite, and Kaylie reveals a reunion plan that isn’t exactly the first thing you’d advise for someone who just got out of intensive mental health care: she intends to prove their parents’ deaths were actually driven by a malevolent supernatural force haunting their father’s antique mirror.

Flashbacks reveal that both children (Annalise Basso as young Kaylie and Garrett Ryan as young Tim) swore to one another they’d prove the mirror’s true nature. Since then, Tim has convinced himself it was all a delusion. But Kaylie is still committed, and the most impressive thing about Oculus’ first half is the ferocious yet methodical drive she brings to the mission.

She hunts down the mirror at a series of auctions and returns it to its original place in the childhood home. There, Kaylie sets up a series of cameras, monitors, and temperature gauges. She’s studied the mirror’s history, and its other victims. She knows dehydration and starvation often occur, so she stocks up on water and food. She knows the mirror kills plants and animals, so she brings in some flora and a dog as test subjects. She figures out the mirror’s radius of influence only extends so far through the house, so there are harbors of mental safety. She even intuits that the mirror possesses a survival instinct, so she rigs an anchor from the ceiling set to a mechanical egg timer – “no electronics, that’s important” – to drop every 30 minutes if one of them doesn’t reset it. So the mirror can’t kill them without sealing its own fate in the process.

Point being, Kaylie doesn’t intend to simply destroy the mirror, but to document its effects to prove her father’s innocence to the world. And after a brief hesitation, Tim comes on board with the plan. So Oculus promises a deadly battle of wits between a demonic force, and these two people, bound by love and a dark past. Good stuff.

Unfortunately, the movie spends its second half undoing that promise. Plot threads – such as their father’s possible affair, and the aforementioned mental safe harbors – are raised and then dropped. The plot ultimately relies on flashbacks to the original murder for momentum, while the scenes with the adult characters a reduced to mere hallucinatory interludes. It becomes ever more apparent the mirror is completely in charge of Kaylie and Tim, and that the filmmakers are completely in charge of their narrative. It’s puppetry rather than an organic story, forced away from genuine agency and character-based decision making and into preordained events headed for a “shocker” ending.

My theory about the “torture porn” genre is that, as dysfunctional as it often is, there’s real potential for unusually primal confrontations with the existential dread that underlies our existence as fleshy, destructible creatures. Similarly for the supernatural horror genre as a whole, it’s the experience of being hunted by a force for which the normal rules of physics, logistical limits, and even the reliability of one’s own perception don’t apply. There’s potential for real artistic and moral substance if deployed right, and towards the right ends. Conversely, deploying this stuff merely for cheap thrills feels gross and nihilistic, precisely because it’s so potent.

Hence my preference for the short film format. If there’s no bloody purpose to it, I can take this kind of thing for 10 or 20 minutes. But stretch it over a feature-length running time, and you damn well better have a point. After sitting through Oculus’ final moments, all I could think was, “You put me through all that for this?”

And actually, now that I do some googling, the film did start out as a short. They should’ve quit while they were ahead.