A man (Jake Gyllenhaal) wakes up on a train sitting across from a woman named Christina Warren (Michelle Monaghan). The woman seems to know him by the name Sean Fentress, but he doesn't seem to know her and appears uncertain of his own identity. After eight minutes, a bomb goes off on the train, and the man awakens to find himself strapped inside a small geodesic dome. There, Air Force Capt. Colleen Goodwin (Vera Farmiga) explains to him through a computer screen that he is actually Capt. Colter Stevens, a decorated army helicopter pilot, now on a mission to locate the maker of a bomb which destroyed a train headed into Chicago. This is to be accomplished using the Source Code, a time loop program that allows him to take over someone's body in a reenactment of their last eight minutes of life.
Stevens has no memory of how he became involved in the Source Code project; his last memory is of flying in a recent mission in Afghanistan while taking on enemy gunfire. Stevens' mission in the Source Code is to assume the identity of Fentress, one of the train's passengers, locate the bomb, discover who detonated it, and report back to Goodwin to prevent the bomber from detonating a larger dirty nuclear device in downtown Chicago, which could cause the deaths of millions of people. Goodwin and the Source Code's creator, Dr. Rutledge (Jeffrey Wright), tell him that the Source Code is not a simulation, but a visit into the past in the form of an alternate reality. He's told that he cannot truly alter the past to save any of the passengers, but that he must gather intel that can be used to alter the future and prevent a future attack.
