Director Rian Johnson is keenly aware that the mechanics of time travel in movies don’t make much sense — and that they make less sense the more you think about them. In Looper, he cheekily dismisses the physics with a wave of his hand and tells you to think about the metaphysics instead.

The scene where this becomes clear is one where old Joe (played by Bruce Willis) yells that “we’ll be sitting here all day making diagrams with straws” when his younger self (Joseph Gordon-Levitt) asks him to explain how either of them can change their shared future. How time travel works is irrelevant.

What matters is what the middle-aged and young version of the same man will do when they’re sitting across the table from each other in a diner, each understanding that their goals and desires are incompatible. Whose wishes should hold sway and which course should the future take?

Young Joe is a “looper” — a contract killer in the year 2044. Time travel has not yet been invented, but it will be 30 years’ time and promptly outlawed. The illegality of time travel doesn’t stop the mob from sending problem people back in time to be eliminated by loopers like Joe because the tracking technologies of their time make it difficult to get away with murder.

The loopers — who are handled by Abe (a scenery-chewing Jeff Daniels), a gang boss from the future — are paid well for their work and lead lives of decadent luxury by the standards of their time. But there is one drawback to the career: every looper will one day need to close his loop by killing his future self. Those that fail to do so face hideous consequences if their mob bosses get hold of them.

When the time comes for Joe to close his loop, his older self escapes and starts to hunt down a child that he believes will grow into a ruthless criminal mastermind of the future. The less you know about what happens next and why when you watch the film the better — but it’s enough to say that the story is riveting throughout.

Johnson has taken great care in devising the plot and the logic of the world it unfolds in. Looper takes place in an understated, organic space that feels like a credible future projection of the world we live in today. The rules for time travel in his world seem rooted more in magic than any pretence towards science, but they’re consistent and believable in the context of the film.

The brushstrokes he uses to convey the social reality and the technology of his future are deft and deadpan — a rainmaking crop duster hovering above a field of Kansas corn, an offhand comment about a retro affectation for 20th century fashion, a contemptuous comment from Joe about guys who use their telekinetic powers to impress girls. There’s little in the way of overbearing CGI — just careful mise-en-scène and subtle detail.

Looper-640 — Hey Joe, where you going with that gun in your hand?

At the heart of the film is Gordon-Levitt’s transformation into a young Bruce Willis. Though the prosthetics and makeup used to make Gordon-Levitt look like Willis are distracting, his performance captures soft-spoken gravel of the older actor’s voice as well his quizzically amused manner. It’s a remarkable feat of mimicry.

Willis, meanwhile, brings his slightly sardonic, somewhat shopworn and self-effacing action movie persona to the film. Hardier, wiser and more resourceful than the young Joe, the older version of the character is the one that dispatches gangsters with street fighting elan in the film’s few exciting action set pieces.

There are some good supporting performances, too, most notably from Emily Blunt as a spirited yet vulnerable mother who emerges as a love interest for young Joe, and Noah Segan as an inept but enthusiastic mob henchman.

Looper is that rare high-concept science-fiction film that manages to walk a playful line between self-importance and self-parody in its philosophical musings. Made with elegance and attention to detail, it blends suspenseful action, a touch of drama, and wry observation with the tropes we know well from films such as Terminator and 12 Monkeys.