A man, portrayed by Jake Gyllenhaal, suddenly finds himself awake on a train, facing a woman named Christina Warren, played by Michelle Monaghan. Struggling with his identity, he learns that he is called Sean Fentress, but he has no recollection of who she is. Just eight minutes into this bewildering experience, a catastrophic explosion rocks the train, jolting him back to reality, where he finds himself confined within a small, high-tech dome. Here, he is confronted by Air Force Capt. Colleen Goodwin, who communicates through a computer screen. She reveals to him that he is actually Colter Stevens, a highly decorated helicopter pilot, trapped in a critical mission to track down the creator of the bomb that caused chaos on the train bound for Chicago.
Utilizing an advanced program known as Source Code, Stevens must inhabit the last moments of a train passenger, reliving their last eight minutes to both identify the bomb's location and the perpetrator behind the attack. With his only memory recalling a harrowing mission in Afghanistan, he embarks on a daunting quest where he learns that he can’t change the past but can gather essential information to protect the future from a potentially catastrophic event—a larger nuclear bomb set to devastate downtown Chicago.
As Stevens navigates through these time loops, every attempt reveals new details, intensifying his mission. He grows closer to Christina, developing a deeper understanding of the human connections he desperately wants to preserve, especially as he faces the inevitable moment of destruction. Despite the insistence from Goodwin and Dr. Rutledge—the mastermind behind Source Code—that he must focus solely on the bomber, he refuses to abandon his desire to save Christina and the other innocents on board.
Through a series of harrowing revelations, Stevens discovers that he is, in fact, deceased. His body was repurposed by the military for the Source Code project after an unfortunate incident in war. This realization drives him to confront Goodwin about his freedom and independence post-mission. Eventually, he identifies the bomber as Derek Frost, leading to the capture that averts disaster in Chicago.
After successfully completing his mission, Rutledge manages to erase Stevens's memories, rendering him a mere tool for future operations. However, Stevens persuades Goodwin to allow him one final opportunity to prevent the train disaster. With the accumulated knowledge from his previous journeys within the Source Code, he successfully disarms the bomb and incapacitates Frost just in time.
In a heart-wrenching yet powerful moment, Stevens shares a kiss with Christina just seconds before the pivotal eight minutes expire. To his astonishment, even after Goodwin complies with his wish to shut down his life support machine, he remains alive within Fentress’s body, enabling him to escape and save the remaining train passengers. In this alternate reality, Stevens manages to write to an alternate version of Goodwin, informing her of his experiences and emphasizing that the Source Code can impact alternate realities. The film closes with Stevens embracing his new life, promising to convey the same comforting message of assurance that Goodwin once offered him: "everything will be OK."
