Colter Stevens (Jake Gyllenhaal) wakes up on a commuter train bound for Chicago; last he checked, he was a Marine fighting alongside his battalion in Afghanistan. The woman seated across from him (Michelle Monaghan) seems to know him, and when he looks into a mirror in the bathroom he sees another man's face. Eight minutes later the train explodes, killing everyone on board and waking Colter again, this time in his own body, locked in a dank cell and talking to a woman on a screen (Vera Farmiga) who's very careful to explain to him as little as possible.

It's an intriguing, whip-crack of an opening for Source Code, a big and twisty sci-fi film that unfortunately only unravels from there. Director Duncan Jones, without the meticulous production design and skilled acting that so distinguished his debut feature Moon, has chosen a bear of a script for his follow-up, littered with giant logical leaps and clumsy dialogue that make the film seem generic at some times and utterly ludicrous at others. Jones brings his skill for sharp visuals and suspenseful pacing, and Gyllenhaal puts in a committed and captivating lead performance, but nothing they do is enough to put the story back on the rails. Where Moon was a marvel of restraint and working within limits, Source Code suffers from a scale no one seems able to control.

At least the basic concept behind the sci-fi is intriguing. The military has figured out a way to tap into the final 8 minutes experienced by a dead person, sending a soldier inside that person for their final moments in order to figure out what killed them. Colter is sent to occupy the mind of a man who was on a commuter train that exploded, and charged with finding the bomber before he carries out a promised second attack; he'll relive those eight minutes again and again until the mission is complete. For a trained soldier, he's not exactly great at the assignment, hunting down obviously false suspects and wasting time chatting up his pretty seatmate. And even with two separate ticking clocks-- the eight minutes on the train, plus the real-time countdown to the second explosion-- the pace of the mission doesn't feel nearly as urgent as it should. But eventually the stakes of the mission change anyway, as Colter gets more information about his situation and takes matters, like so many action heroes before him, into his own hands.

The best example of Source Code's lack of cohesion is Jeffrey Wright, as the mysterious and amoral head of the government agency that has sent Colter Stevens on his time-traveling mission to this doomed train. Affecting an accent and swanning around the high-tech office like some foppish despot, Wright may just be trying to make the most of dummy lines like "Now we finally have a powerful weapon in the war on terror," but he's completely at odds with the grim, realistic performances from his co-stars. Then again, he's at least more interesting-- Monaghan is pretty but very blank as the accomplice Colter meets then falls in love with rather suddenly on the train, while Farmiga, perched behind a computer monitor for nearly the entire film, seems merely to be cashing a well-deserved paycheck.

It's not that Source Code is unwatchable, or that some relatively enjoyable sci-fi hokum isn't welcome, especially when it looks this good and moves along so pleasantly. But Jones's previous film had definitely taught us to expect better, and especially since Moon's script was also fairly flawed, it's a shame to see him make so little of what he was given here.