Colter learns he's part of a government experiment called the Source Code, making him to live the life of another.
As Colter lives Sean's final moments, he becomes more certain that he can prevent the first tragedy from occurring as long as he doesn't run out of time.
Jones proves with Source Code that he understands how to pace a traditional thriller, and Gyllenhaal's display of comedic chops, confident bravado and tender sympathies prove that the 30-year-old actor has arrived at leading-man status.
Source Code is a mind-bending minor classic of its type, a sort of Groundhog Day with terrorism and fewer weather reports. It's as if The Matrix hopped aboard a rapid-transit line and got derailed into the American military bureaucracy.
Ultimately, Source Code makes good on its Hitchcockian opening sequence -- it may pale in comparison to the master, but it's a fun, puzzle-filled ride, with excellent pacing and a mounting uneasiness that recalls the best episodes of The Twilight Zone.