Help me understand a movie (its a good one, possible spoilers)

Feb 23, 2003
Ok, movie: Donnie darko

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Well, i love movies like this, this one was specacular!

I need help understanding one thing, but first ill tell you i believe this is the correct answer, i just wanna make sure/hear other opinions.

Ok, at the end, when the engine hits again, and he is laughing in bed, is it that he has seen his own future, that his own fictitious god has laid out for him, and he has been shown the future to be given a choice? Like, if he chooses to follow franks advice and leave, Gretchen will die, but if he stays in his bed and accepts death, she will be saved, and the "world" will not end....

Btw, following this movie with the american beauty soundtrack is key :)