The Primer.

“Primer” is a raw, low-budget take on that staple of filmed sci-fi, the time travel tale. But unlike movies that take you through time on a DeLorean, or on a comfy chair with attached roulette wheel, or simply through the power of wishing really hard, the time machine in “Primer” is built (partly) with present-day technology, and it looks and feels like something that could exist in the real world.

The movie achieves verisimilitude through an almost documentary-like approach, making it one of the most engrossing time travel movies ever told… for the first hour, at least. After that point, it devolves into a confusing morass of causality loops and overlapping timelines that demands multiple viewings. However, it’s unclear just how many viewings are required before the movie’s third act even begins to make sense.

Abe (David Sullivan) and Aaron (writer-director Shane Carruth) are entrepreneurial scientists, working on experiments in Aaron’s garage, hoping for the big breakthrough that will allow them to sell the patents and become millionaires. I doubt most viewers will understand exactly what they’re trying to accomplish here, or all the scientific jargon they use, but this only adds to the realism.

Abe and Aaron create a boxy contraption from wires and sheet metal and car batteries that seems to take on a life of its own, and produce curious results. They discover that a piece of chocolate briefly placed inside the box shows signs of being inside for years, not minutes. Eventually, they figure out that objects placed within the box are actually looping between two points in time: the time the machine was turned on, and the time it is turned off.

They immediately set to work building larger versions of the box that can accommodate people. And one of the unique aspects of “Primer” is that time travel is never shown to be as easy as pulling a lever or stepping through a gateway; a time traveler attempting to go back one day must actually lie inside a dark, claustrophobic compartment for an entire day.

During their initial forays into the past, Abe and Aaron take elaborate measures to avoid meeting their former selves, or otherwise tampering with past events. Neither of them ever comes out and says “paradox” or “causality loop”, because another great thing about this movie is it is assumes you’re already familiar with these concepts from a thousand other time travel stories.

Soon, the men start trying to profit from their time travelling adventures in obvious ways, like using foreknowledge of the stock market to make money day trading. They even start to mull over betting on sporting events, or even getting rich off the lottery.

But then a tragedy befalls one of their friends at a party, and Aaron becomes obsessed with using their invention to stop it from happening in the first place. And this is where the film takes a hard left turn, and the plot begins to disappear before our very eyes.

It seems there’s another time machine that’s been running all this time, which allows the characters to travel even farther back than previously established. And then we find out it’s possible to fold up the time machine and put it inside another time machine, allowing them to travel even farther back. The final twenty minutes of this movie devolve into convoluted scenarios involving doppelgangers, potions and poisons, multiple versions of Abe and Aaron drugging each other, and future selves taking the place of past selves.

And it’s never clear exactly what happens at the party, or why Abe and Aaron would use a time machine—a device that could potentially change the course of human history (in more ways than one)— for the sole purpose of getting involved in a relatively inconsequential event.

Given that the first hour is so compelling, it’s a mystery as to why the final twenty minutes are so confounding. Did they simply lack the budget to film crucial expository scenes? Or was director Shane Carruth deliberately being obtuse for the sake of generating repeat viewings? I would guess it’s a little of both.

Others have written lengthy dissertations about this movie, where they chart all the many timelines, and pick up on the subtle clues that show us characters have traveled through time much more than they’re letting on. And I’m sure it all makes sense, if you have the time and energy to read and digest an entire manifesto about a single film.

Ultimately, a movie exists to entertain in the timeframe in which it is being watched. The script may be a finely-crated intellectual marvel that will be studied in film schools years from now, but to those who watch and enjoy movies from their hearts and their guts, the final act is a huge letdown. As it stands, “Primer” is one of the best two-thirds of a sci-fi movie ever made. By all means, see it, but don’t expect to make any sense of the ending.