Terminators travelling in right direction

Killing time: Joseph Gordon-Levitt stars in 'Looper' (AP)

Rian Johnson's time-travel thriller springs us into the future, but for anyone with a memory for movies it's another trip back into the past. Imagining the future is the oldest hat of the lot, its great virtue being the catch-all absolution of narrative responsibility. Got a problem with leaky logic or crumbling plausibility? Don't worry about it – we are in the future.

Try and get your head round this. It's America 2044, sunk in an economic blah where crap cars and vagrants throng the streets. Thirty years hence, time travel will have been invented. It will also have been made illegal, exploited only by crime syndicates that dispatch their enemies back to the past where trained assassins known as "loopers" will murder them, safe years away from detection. The anonymous victim, hooded and cuffed, arrives out of thin air, the looper will shoot him dead, then collect his fee of silver, strapped in handy briquettes to the victim's body. One such looper is Joe (Joseph Gordon-Levitt), a smartly dressed young fellow who's well into his routine: single gunshot to the victim in a lonely cornfield, a cup of coffee at a favourite diner, burn the corpse, then back to the city for some R&R –girls, clubs, drugs (dispensed in eyedrops now, which must play hell with your contact lenses).

Trouble starts for Joe when his looper pal Seth (Paul Dano) does the unforgivable and lets a victim escape. Seems the victim this time was Seth's older self, sent back from the future to get whacked, a ritual known as "closing the loop". That's quite some ritual when you think about: isn't an assassin killing his future self a pretty enormous conflict of interest? Why don't they send someone – anyone – else to do the job? Joe's boss (Jeff Daniels) looks like he will explain the conundrum at one point, then decides it might slow up the plot and just says, "Time travel fries your brain like an egg." It would appear that Seth's luck is out, and that Joe had to shop him. Karma arrives with due dispatch when Joe's lining up his next hit in the cornfield, checking the time on his non-futuristic pocket-watch, and who should arrive marked for slaughter but – yippee-kai-ay, motherfucker! – Bruce Willis.

The twist being that Bruce is Joe's own older self, returning us to the problem outlined above. Why would you want to murder your fiftysomething self? It does explain, though, why Joseph Gordon-Levitt's appearance has been digitally meddled with, his brow, eye-colour and nose shaped to conform with his older ego Bruce. It's not the greatest match-up you'll ever see, not a patch, for instance, on Josh Brolin's eerily right impersonation of the younger Tommy Lee Jones in Men in Black 3. Joe senior, escaping his execution scene, goes on the run to find the "holy terror" of the future, a killer named the Rainmaker who murdered Joe's wife. The second half of the film switches from town to country as Joe seeks refuge at a lonely farm where homesteader Sara (Emily Blunt) lives with her little boy (Pierce Gagnon) and a big shotgun to scare off the city scavengers. Emily Blunt continues her good year in movies (The Five-Year Engagement, Your Sister's Sister) with this defiant-heroine turn.

Looper has its shares of plot-holes and design flaws, yet somehow just keeps going, blatting away your doubts. The conviction of the cast is vital, for the film's secret weapon is not its cleverness, but its heart.