A horror movie has one job really: be scary. Beyond that, most flaws can be overlooked (at least temporarily). 'Oculus,' a horror movie ostensibly about a haunted mirror, has many flaws (not the least of which is that it's a horror movie about a haunted mirror; a tired premise if there ever was one), but it does one thing very, very well: be scary.

Based on a 2006 short film of the same name, 'Oculus' stars Karen Gillan ('Guardians of the Galaxy') and Brenton Thwaites as Kaylie and Tim, siblings still haunted by the death of their parents when they were younger. Now grown (with Tim just finally being released from a psychiatric hospital), the two reunite to confront an ancient mirror they believe is responsible for the violent end of their parents.

There are very few scenes that take place outside of the confines of their childhood home, where Kaylie has set up an elaborate test/trap to prove the mirror is haunted and ultimately destroy it. This gives the film a tight, claustrophobic feel; the characters wind up being trapped in the house and the audience really feels it. Because the film is so small and the characters extremely limited, we are able to connect with them on a human level, so once the stuff hits the fan, we're emotionally invested.

Particularly strong are the film's female characters: Gillan's Kaylie and Katee Sackhoff, who plays her doomed mother. Gillan brings a quiet obsession to Kaylie, a woman with some serious emotional baggage bubbling just below the surface. She thinks she's strong and smart enough to take on an evil mirror, but quickly finds herself overmatched and her slow breakdown is one of the film's pleasures. Without giving too much away, Sackhoff has a number of roles to fill, from doting mother to jealous wife to ... well, you'll see ... and tackles them all ably.

Shot almost like a one-act play, 'Oculus' takes place in both the past and the present, allowing the film's mysteries -- What happened that drove Tim crazy? What really happened to their parents? What is actually real? Are either of our narrators even reliable? -- to unfold concurrently. As the film propels itself forward, director Mike Flanagan intercuts the escalating tension in both timelines, blurring the line between past and present and reality and imagination. You never really know what's coming, yet Flanagan handles the material with such an assured hand, you don't leave confused either.

The one mystery that 'Oculus' wisely avoids is the one about the mirror itself. The mirror is haunted, f---s you up and that's all you really need to know. Too many horror films of late try to shoehorn in unnecessary mythology in the third act that slows down the plot and is largely useless beyond trying to set up a future franchise. We never learn what it is the mirror does or why or what insane black magic warlock created it and that's fine. 'Oculus' is smart and confident enough to let the audience's imagination run wild with what the mirror is actually doing and why.

Listen, we could spend a lot of time talking about the duality of man and the evil within ourselves, but what you really want to know is whether 'Oculus' is scary. And, boy is it. It's not overly gory (though there is certainly some blood), but the film doesn't need it to freak you out. There's little, if any, CGI, no found footage and no corny jump scares. 'Oculus' is remarkably free of horror gimmicks, knowing that it can and will freak you out on its own.

While there are moments of horror tropes you've certainly seen before -- the haunted mirror, the family slowly driven to violence by a menacing force -- 'Oculus' proves that the devil is in the details. Quite literally.


'Oculus' opens in theaters on April 11.

Mike Sampson is the Editor-in-Chief of ScreenCrush.com.

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