Colter, like Deckard before him, has to accept who and where he is, even as these definitions recede before him.
Blade Runner is the be-all and end-all for me in science fiction.
-- Duncan Jones
I've seen things you people wouldn't believe.
-- Roy Batty
Captain Colter Stevens (Jake Gyllenhaal) wakes on a commuter train to Chicago. Unsure where he is, he turns to the girl in the seat across from him, Christina (Michelle Monaghan), who calls him "Sean." He insists on his own name, becomes increasingly anxious, doesn't appreciate her joking ("You okay there, 'Captain?'"). He makes his way to the bathroom, where a glimpse in the mirror reveals that he's not himself, literally. Even when his wallet ID confirms he's this guy Sean, a teacher, Colter resists. "I don't know who Sean is and I don’t know who you are," he tells Christina. She assures him, "Everything's gonna be okay." She's right and she's not right.
The train explodes a moment later.
As Colter soon learns in Source Code, he is, in fact, Colter. But he's not exactly on this train and he doesn't know Christina. He's an Air Force pilot, on an experimental military mission that involves time travel and his insertion into someone else's body. His handler, Goodwin (Vera Farmiga), appears to him on a monitor, as he's strapped into a capsule, the dark and gritty kind, not sleek and shiny. Where he's landed, or is about to land, is unknown. Goodwin tries to get him focused, explaining that he's on a train bearing a terrorist's bomb, and it's up to him to discover the bomber's identity. He has eight minutes -- again and again, until he gets it done.
While the time travel details of Ben Ripley's script are convenient, the tension it establishes is compelling, mostly because it's premised on Colter facing himself, repeatedly, and in various situations. In this, the film isn't so unlike the idea that made director Duncan Jones' first film, Moon, so terrific. Where that movie had an astronaut-grunt played by Sam Rockwell realizing his own always-already existential loneliness, the new one puts Colter into what seems a caffeinated version of same. That's not to say that Colter is only alone, but rather, that he comes to appreciate what that means and how he might help someone else not to be.
This means that Colter, like Deckard before him, has to accept who and where he is, even as these definitions recede before him. The motion of the speeding train helps to make this situation seem pressing, as does the Groundhog Day-like repetition of events. The conductor comes to collect his ticket, a passenger spills coffee on his shoe, another passenger drops a wallet, and yet another uses a cell phone. It's not clear which event is important or if any is, though Colter is instructed that this bomber has something to do with a bigger, slightly later explosion in the city. It's too late, he's told, to stop the explosion on the train, but he can stop the next attack.
As Colter develops something like a relationship with Goodwin, he grows frustrated at her insistence on the one task, and demands to be debriefed. When he sees the man in charge, the "source code"'s inventor, Dr. Rutledge (Jeffrey Wright), Colter intuits that he must expand his mission parameters. The researcher has a crutch and a lab coat and a muttery affect, exactly the signs that he's lost sight of human dimensions. Sitting across from Christina again and again, these are exactly what Colter sees ("You're real," he says, an observation she actually appreciates). And so he proceeds through his steps, figuring how to get around what he's supposed to be doing in order to do the right thing.
The more Colter's managers resist his going off-task, the more he's justified in doing so, at least in the movie's moral universe. While Rutledge is stereotypically depraved, Goodwin serves as a slightly more complicated foil. “The source code is a gift," she says early on, "Don’t squander it by thinking.” That's enough to get his wheels turning, of course, and soon he's embodying the film's contemplation of the military's functions in a world where civilians remain safely clueless and soldiers follow orders.
While Colter comes from a military family and believes in his service -- last thing he remembers, he was flying sorties in Afghanistan -- the film submits that thinking is something of a gift in itself. Colter's self-reckoning takes a mostly conventional structure, wherein he finds that careless superiors exploit unconscious grunts (see also: Alien, Jacob's Ladder, The Jacket), his outrage righteous and poignant. It helps that he's on a speeding train of a movie, one that assumes you know most of this anyway.