As long as we're making comparisons, we should get a few more out of the way, namely, The Matrix, Inception, Deja Vu, and TV's Quantum Leap. (And probably a few more that are escaping me at the moment.) Still, out of elements that are not entirely fresh, Source Code forges its own compelling story.
Colter Stevens (Jake Gyllenhaal) snaps to attention on a train, sitting across from Christina (Michelle Monaghan), a woman who seems to know him, but calls him by a different name. Colter is disoriented and confused; he has no idea where he is and how he got there.
Eventually, Colter learns that he's on a military mission to uncover the identify of a bomber on the train; the bomber is believed to have targeted Chicago for a terrorist attack, and time is of the essence. Through a blitzkrieg explanation -- something about quantum mechanics, muttered by a brilliant scientist / civilian (Jeffrey Wright) overseeing the mission -- Colter is told that he can be sent back into the mind and body of a man on the train, but only for 8 minutes. The mission's overseers, will keep sending him back for the 8-minute loop, and he must piece together enough clues to be able to catch the culprit. He's fed only a limited amount of information before each 8-minute loop, through the calm, urgent voice of military officer Carol Goodwin (Vera Farmiga).
It's best not to think too much about the HOW; it's a classic McGuffin, a device that exists mostly just to set the plot in motion. And that's probably more than needs to be said about the story. As with Moon, Duncan Jones' directorial debut, the less you know in advance, the better. The discovery phase is half the fun.
What is thrilling and exciting and rewarding is that Jones proves that he's no one-trick pony. Source Code moves like a horse with rabies; sometimes it's ungainly, sometimes it's a bucking bronco, and sometimes it quiets down to catch its breath. There are missteps here that were not present in Moon: for one thing, there's probably too much narrative exposition at times, and a propensity for sentimentality is indulged to a greater degree than truly warranted. But not enough to derail the film. (Sorry, train metaphors are difficult to avoid entirely.)
What Source Code shares with Moon is an intelligent treatment of material that could have been, in lesser hands, a simple "boom bang" movie. The script, credited to Ben Ripley, structures the story as a puzzle, which is mind-bending at times and appears to leaves gaping holes at others. What appear to be loose threads, though, are eventually all woven up tightly, which help make the film feel contained within itself and wholly satisfying.
Frankly, "boom bang" movies are fine in their place. For viewers who want explosions and noting else, Source Code delivers thrills and chills. And for everyone else who'd like a bit more for their time and money, like food for thought and characters who act like human beings, Source Code satisfies those cravings as well.