On a commuter train heading into Chicago, a young man jolts awake in the middle of a conversation with his girlfriend, Christina (Michelle Monaghan, “Due Date”), and clearly has no idea who she is, where he is or how he got there.

He excuses himself to the restroom and doesn’t recognize the face in the mirror. A bomb goes off, blowing the train and its passengers to hell and gone. He wakes up again in a small chamber with the face of female Army officer Goodwin (Vera Farmiga, “Orphan”) looking at him via monitor.

He learns he is really Colter Stevens (Jake Gyllenhaal, “Love and Other Drugs”), an Army helicopter pilot stationed in Afghanistan, and — pay no attention to the scientific explanations, because life is too short to attempt figuring them out — he is part of Project Source Code, which allows his mind to enter the body of a Chicago teacher eight minutes before the bomb explodes.

This gives him 480 seconds to find the bomber. If detonation occurs before his mission is complete, he will be sent back to the train for another eight minutes of searching. The hope is that he discovers the identity of the terrorist so a second bombing, this time nuclear in nature, can be prevented in downtown Chicago.

Colter is told that he cannot change the events of history, and that the train must explode no matter what he does, which complicates his efforts, because after several eight-minute passages, he begins to fall in love with Christina. Goodwin develops sympathy for his situation, although the man responsible for Project Source Code (Jeffrey Wright, “Cadillac Records”) may not be trustworthy.

The suspense mounts with what appears at first to be every visit to the past, as Colter learns a little more each time and eliminates passengers from suspicion, one by one. What seems to be an inevitable ending gets complicated when we realize that Colter isn’t really time-traveling, but actually creating a new alternate reality each round.

I know it sounds confusing, but it’s all a lot easier to figure out in a 90-minute film than in a 500word review. Just lean back, grab the armrests and go along with the theoretical science.

Directed by Duncan Jones (“Moon”) and written by Ben Ripley (“Species: The Awakening”), “Source Code” is a terrific thriller with an outstanding cast and an ending you probably won’t see coming. Gyllenhaal more than makes up for his last two disappointments, while Farmiga builds on her recent successes.

If you’ve grown more than weary of the pap “sci-fi” of Spielberg and Lucas, give this one a try.