SONY PICTURES ENTERTAINMENT/AP PHOTO

This film image released by Sony Pictures shows Joseph Gordon-Levitt in a scene from the action thriller "Looper."

It's distracting at first: the fact that you're looking at Joseph Gordon-Levitt but he doesn't look exactly like the Joseph Gordon-Levitt you've come to know and love. Aren't his eyes brown? Isn't his nose longer and thinner? Even the blasÈ smirk on his face seems like an unfamiliar expression given his usual likable, boyish cool.

Then Bruce Willis shows up, and you realize, a-ha! Gordon- Levitt, tweaked slightly through blue-green contact lenses and prosthetic makeup, is essentially channeling Willis because they play the same character reunited in the future over 30 years of time travel. It's not a dead-on impression or even a parody, and it's not meant to be; this is not Josh Brolin doing a younger version of Tommy Lee Jones in the most recent "Men in Black" movie. But the sighs and the cadence and the general persona are there.

So now that we've gotten that out of the way, we can focus on the really mind-boggling stuff.

We haven't even begun to explain the premise of "Looper" yet and your head is probably already starting to hurt - in a good way. It's worth the effort. Fans of time-travel movies know that much of the fun of the genre comes from obsessing over whether it all makes sense, both while you're watching it and in long, complicated conversations afterward.

"Looper" makes sense ... I think. I've got a couple logistical questions. But what's smart about it - and what makes it more compelling than colder sci-fi - is the way writer-director Rian Johnson establishes the machinery of the time-travel concept, then steadily pushes it into the background in favor of exploring his characters and the difficult questions they face.

Johnson's feature debut - 2005's "Brick," a verbally stylized film noir set in a modernday Southern California high school, also starring Gordon- Levitt - signaled him as an ambitious filmmaker with a distinctive voice. Here, with his third film, he's expanded both his scope and his eye for vivid detail. He incorporates a variety of genres and influences, from dystopian, futuristic science fiction and dark comedy to parental drama and romance, with a Wild-West shootout and even some "Terminator" thrown in. But he always stays true to his characters in his fully realized world.

The beginning of "Looper" looks like something that might best be described as "Les Miserables" meets a Lancome ad.

The year is 2044, and America has fallen into a state of stylish squalor. Gordon- Levitt's character, Joe, is a junkie and former criminal who makes ends meet in this depraved world by working as a "looper," a hired gun. (Paul Dano plays his troublemaking co-worker.) Time travel hasn't been invented yet, but it will be 30 years further in the future. A powerful mob boss known as the Rainmaker sends his enemies back in time to have them obliterated with no bodies to dispose of and no loose ends.

All Joe and his fellow loopers have to do is stand in a certain place at a certain time and the victim will show up, hooded and kneeling. One quick blast and it's over. But sometimes, future versions of the loopers themselves show up on the spot; this is known as "closing your own loop," and it means getting a handsome payout and a set period of 30 more years to live it up. Trouble is, when Joe's loop arrives in the form of Bruce Willis, he hesitates, then watches as his future self runs off.

Although they're the same person, decades of life experience have put them at crosspurposes, and in a dazzlingly clever nugget of a concept, each is hunting the other. The Willis version of the character wants to stop the Rainmaker when he's just a young boy so that he may enjoy the happy life he's fought so hard for; to achieve this goal, he makes some choices that many in the audience will find unsettling. But the Gordon-Levitt version is so selfish, he simply doesn't care - he just wants this old man to die already.

The scene in which they meet at a diner and spell out what they want over plates of steak and eggs is both thrilling and darkly funny. This is perhaps the most flawed character Gordon-Levitt has played, but there's always great honesty and humanity in everything he does. And while Willis gets to flex his action-star muscles, it's the vulnerability and worldweariness of his performance that's even more appealing.

The introduction of Emily Blunt, as a single mother seeking refuge from big-city life on a farm with her strikingly gifted young son (Pierce Gagnon, with tremendous presence beyond his years), adds another emotional layer to this story. It softens and slows the film down but that's not necessarily a bad thing; it's just one more example of how "Looper" keeps changing effortlessly and taking you to unexpected places - past, present and future.

"Looper," a TriStar Pictures and FilmDistrict release, is rated R for strong violence, language, some sexuality/nudity and drug content. Running time: 119 minutes. Three and a half stars out of four.

Motion Picture Association of America rating definition for R: Restricted. Under 17 requires accompanying parent or adult guardian.