Shakefire writes, "Eight minutes. That’s the amount of time you have to find out who planted a bomb on a subway train headed for Chicago before it explodes, killing everyone. The good news is that you can infinitely repeat the same eight minutes until you find the bomber. The bad news is the event already happened and everyone died in the explosion.
Source Code is an experimental new procedure that allows a subject to essentially hack into the final memories and experiences of someone and take control of his or her persona. Captain Colter Stevens (Jake Gyllenhaal) has been recruited by the government to use Source Code to go back to the events that day and discover the identity of the person responsible so they can stop future attacks."