Minority Report meets Groundhog Day as soldier Jake Gyllenhaal finds his mind in another man's body on a train that will be blown up in exactly eight minutes, killing everyone on board. After the event, he is jolted back to a strange chamber where a military handler (Vera Farmiga) informs him the scenario will be repeated until he finds the bomber, thus averting an even greater atrocity. Is it a simulation? A dream? A crazy experiment? Fellow passenger Michelle Monaghan inspires him to work it out - fast - as director Duncan Jones follows his cult debut Moon with another sci-fi cracker.
Using quantum theory, parabolic calculus and parallel realities to address issues of identity, mortality, morality, Source Code sounds like the sort of impenetrable brain-fart you'd only expect to hear from the Architect of The Matrix.
Miraculously, Duncan 'Zowie Bowie no more' Jones has built on the Bafta-winning promise of Moon by turning the idea into a sharp and compelling mind-bender that goes easy on the gobbledygook.
The on-form Gyllenhaal is Colter Stevens, an army helicopter captain who suddenly wakes up in the body of a teacher called Sean Fentress on a train bound for Chicago. But aside from a little agitation, his travelling companion Christina (Monaghan) clearly sees nothing amiss... until the train explodes.
The shellshocked Colter immediately finds himself in a cockpit-like chamber, where a uniformed woman he remembers as Goodwin (Farmiga) debriefs him via a monitor.
If this a training exercise, Goodwin and her superior Routledge (Jeffrey Wright) are taking it extremely seriously. They believe the train was just a warm-up to the bomber's main target: the entire city of Chicago.
Like the rest of the passengers, Sean Fentress is dead. But, like the afterglow from a light bulb, his remaining brain activity was harnessed and incorporated into Routledge's revolutionary 'source code' which allows an active brain - like Colter's - to relive his last eight minutes. Over and over again.
So it's Colter's duty to keep going back until the terrorist is unmasked. Unfortunately, source code can't bring the dead back to life - the event has already happened. But while he's not there to make friends, his mission is still a bruising process of trial and error.
In the eerie breathers between each short race against time, Colter gradually uncovers the truth about his own situation... and the dark side of source code.
As with most time travel escapades (or time-displacement, as Routledge would have it here), it throws up as many questions as it answers.
Pleasingly, Jones and screenwriter Ben Ripley fill more plot-holes than most. They also maintain a suspenseful head of steam while allowing jaws to unclench every now and then with shots of wry humour.
With the emphasis on mystery over sci-fi, effects are used sparingly but make their point, the highlight being when Colter jumps from the speeding loco in a single shot that cleverly blends real stuntwork with CGI.
Impressive as it is, the Jones-Ripley theorem is not perfect. The on-board mystery could have been teased out more and, after reaching an ideal stop, the film overshoots towards a cosier coda a la Minority Report. It would have been bolder and more in keeping with what went before to end in ambiguity, Inception-style.
Ultimately though, sweating the small stuff simply reinforces the overarching message: life is short. That being so, this is time well spent.