HUGE SPOILER ALERT:If you haven't seen the movie, this will absolutely ruin it for you. If you watched the movie “Source Code” and walked away thinking, “Hey, that was just like Groundhog Day, but not funny,” we can’t really blame you.
Most movies involving alternate realities ask that you check your brain off and don't bother to pick it up on the way back out. Some movies, though, can't even follow their own logic for more than a few minutes without getting hopelessly confused and ending up like a dog chasing its own tail. Through a portal. Unlike the elegant, if somewhat obvious, ambiguity of the movie "Inception," the directors of "Source Code" decided to leave the movie hanging with not one, but several possible explanations for the ending, each one somehow more depraved than the last.
First, let's recap the main plot points, ignoring as we go any inconvenient "science" that might get in the way.
A terrorist blows up a commuter train in Chicago. This, according to intelligence reports, is just a precursor to a larger attack involving a "dirty bomb," apparently planned for later that day. The military manages to scrape enough brain matter up from one of the passenger to use for their "Source Code" project, which retrieves the last eight minutes of memory stored in the brain.
The memories are inserted into the brain of a test subject for the Source Code project, Captain Colter Stevens, an Army helicopter pilot who is clinically dead, except for the part of his brain that can interact with the eight minutes of someone else's memory. And also the part that can interact with the video feed of the scientists manipulating his brain. And the part that can create a virtual environment for him to exist in when he isn't jacked in to the other guys' brain, presumably as a coping mechanism. So not very dead at all, but that pesky fact doesn't really seem to bother anybody else.
Colter is then tasked with finding the bomber by reliving the events of the passenger whose memories have been inserted/forcibly thrust into his own. This he is able to do by altering the events in the victim's memory, repeating as necessary. His mind, though, begins to experience adverse effects from the shock of living in someone else's reality, also probably from getting his ass blown up repeatedly. Stevens is informed several times that altering the events in the memories will have no outcome in the real world, since the events have already occurred. This point is emphasized, in big bold letters. [b]NOTHING HE DOES WILL HAVE AN EFFECT ON THE OUTCOME.[/b] Like that. Wonder why?
Colter finds the bomber, but along the way falls in love with a fellow passenger and saves her from the bomb, hoping that in real life she will be saved too. She isn't, but the bomber is stopped in real life.
When he fails to save the girl and discovers he is "mostly" dead in the real world, Colter asks to go back in a final time, and after the eight minutes is up, have his life support terminated. He saves the entire train, stops the bomber, calls his dad and kisses the girl, then waits to die.
BUT after the plug is pulled, Colter is still alive! What is this madness? The so called "source code" is actually a portal to another reality, an alternate dimension where he can live out the rest of his days happily with the girl from the train. Everyone is happy. Roll credits. Oh, and they also end up at the Cloud Gate sculpture , aka "The Bean" in Chicago, which Colter had "flashbacks" of while he was popping in and out of the other consciousness/dimension. Which means that, uh, fuck it. We have no idea.
I am totally the guy you think I am. Totally.
Colter has to live for the rest of "his" life in a different person's body. Never mind that he has to assume a new identity, teach school instead of fly helicopters and God knows what else to maintain the lie that his life has become. Whenever he looks in a mirror, he is going to see the image of a dead guy. Who is not the dead guy that he is. In that other dimension. That's just gotta screw with your mind. He also has to lie to the girl, every minute of every day, and know that when she gazes at him adoringly, it isn't him she's seeing. If he ever tells her, her brain will absolutely explode. Bear in mind, this is the happiest scenario possible.
Keep in mind that Colter will absolutely go insane at some point in the future. He already suffers from post-traumatic stress disorder, after you know, dying in a helicopter crash. The awakening of a realization that he is "mostly" dead very nearly causes his brain to melt down. He manages to gut it out and complete the mission, but only after being blown up several more times.
The guy who is now inhabited by Colter Stevens' not-at-all dead brain, Sean Fentress, died in "real life," so it's possible we're not supposed to care what happens to his walking corpse. Stevens inhabits Sean's body in the magical new dimension he discovered, which means that both Colter and Sean are dead in the other dimension, along with everyone else who died on the train. But that's OK, because they get a great new, bomb-free existence. But not Sean. Sean is not only dead; he is wiped from existence, bumped from the astral plane by Stevens. Except that his brain is still a part of Colter's consciousness. Colter can access Sean's memories for the last eight minutes, which means that Sean must be alive somehow, at least a little bit, struggling futilely against some random asshole who has taken over his body, his existence, and the hot chick on the train that he never got to sleep with. Hell couldn't be much worse than that.
There is no God or at least not in the concept that we can imagine. It means we can leave this existence and start a new one. The caveat is, you have to do it through hijacking someone else's brain. This can only lead to a massive spike in human brain trafficking, especially among recently deceased celebrities. We can also cheat death, although we've learned from 18 "Final Destination" movies will eventually result in us being impaled by a 200-foot crane or something equally plausible.