In Source Code, Colter Stevens is trying to locate a bomb on the commuter train. The bomb is hidden inside the train's lavatory, and finding it is one of the first major steps in his investigation.
Stevens repeatedly relives the final eight minutes before the train explodes. At first, he believes the mission is a training simulation, but he eventually learns that the explosion really happened and that authorities need him to identify the bomber before a second attack occurs.
Finding the bomb does not solve the mystery by itself. Stevens must also determine who planted it, which leads him to investigate passengers, search belongings, and follow clues across multiple versions of the same event. The terrorist is eventually identified as Derek Frost.
The film's title refers to an experimental system that reconstructs a recent past event from the memories of people who died in it. The story therefore combines a traditional bomb-disposal mystery with science fiction involving memory, identity, and alternate outcomes.