"Oculus" is about one adjustment away from being a superior thriller. The screenwriters spend most of the movie painting themselves into a corner, and the audience waits to see how they will get out … except they never do.

Just one idea — one medium-size idea — might have saved it and turned this movie from a well-made curiosity to something everyone should see.

"Oculus" has everything else: a good premise, smooth and skillful direction, and a leading lady in Karen Gillan who has a vivid face and impressive verbal dexterity. Listening to her tearing through the text, talking fast and with authority, you know instantly that this is someone who was conservatory trained. And despite the American accent, it comes as no surprise to find out later that she was born in Scotland and schooled in London.

A brother and sister do battle against the evil mirror that destroyed their family. That’s right, a mirror. It’s floor-length with an ornate black frame and has been around since the 16th century, wrecking people’s lives. In 2002, it caused a wife to lose her mind and a husband to commit murder. Now, years later, the grown children of the hapless couple move the mirror into the house where the tragedy took place and, surrounded by video cameras, set out to prove that the mirror was at fault, not their parents.

‘OCULUS’ Rated: R Opens Friday

That’s both the gimmick and the story of "Oculus," and it’s quite enough. As the film goes on, we come to understand the mirror’s powers, its range of influence, its ability to regulate temperature and to cause hallucinations. All the details serve to create a texture of reality around this mirror, which is some kind of portal into absolute darkness.

Director Mike Flanagan, who co-wrote the screenplay, intercuts the events of the two fatal nights — the one in the present and the one experienced by the protagonists as children back in 2002. In the beginning these flashes to the past are traditional flashbacks. We know when they begin; they last for a couple of minutes, and then we know where they end.

But gradually, the flashbacks accelerate in frequency, to the point where 2014 and 2002 are switching back and forth from one shot to the next. "Oculus" becomes filmically sophisticated, juggling real past events, real current events and past and present hallucinations, all seamlessly, but better than seamlessly — with an emotional logic that adheres to no set pattern, just an unerring intuition.

Whose intuition should be praised? The screenwriters? The director? The editor? Or everybody? It’s hard to tell whether all this intercutting was part of the original conception or something arrived at after the event; I’d guess the former. Either way, it’s this movie’s claim to fame, and "Oculus" would be worth its fame — more fame than it’s going to get — if it all didn’t come crashing down with a cliched ending.