Imagine that you are in darkness, in almost a complete stage of unconsciousness as you look through a porthole to another world of memories. In this world, you have a life, a family, friends, a career, dreams, wishes--all the stuff that comes with your average lifetime. You are smart, know exactly where you are, who you are, and everything is so very real. Your life in this world is completely oblivious to that other world, the not-so-nice one outside that's cold and dark.
Then imagine that suddenly, from the darkness, comes a loud...not quite a boom, but more like someone just punched your ear really hard--but it's from the outside. You sense that something is wrong. So you pull away from the porthole, emerge from the darkness...
And you open your eyes.
All that you can see is a sterile, empty place, devoid of any other life. Oh, and you're in a little capsule-pod thingy, and you're completely naked except for some weird green gooey stuff.
Still interested? Here's the rest of what happens.
And if you already know how that goes, the why part's also helpful.