Ryan at The Reader’s film blog has the official plot to the Green Lantern movie, and it seems to put about 87 carts before the green horse:
(basic plot spoilers after the jump)
In a universe as vast as it is mysterious, a small but powerful force has existed for centuries. Protectors of peace and justice, they are called the Green Lantern Corps. A brotherhood of warriors sworn to keep intergalactic order, each Green Lantern wears a ring that grants him superpowers. But when a new enemy called Parallax threatens to destroy the balance of power in the Universe, their fate and the fate of Earth lie in the hands of their newest recruit, the first human ever selected: Hal Jordan (Ryan Reynolds).
Hal is a gifted and cocky test pilot, but the Green Lanterns have little respect for humans, who have never harnessed the infinite powers of the ring before. But Hal is clearly the missing piece to the puzzle, and along with his determination and willpower, he has one thing no member of the Corps has ever had: humanity. With the encouragement of fellow pilot and childhood sweetheart Carol Ferris (Blake Lively), if Hal can quickly master his new powers and find the courage to overcome his fears, he may prove to be not only the key to defeating Parallax…he will become the greatest Green Lantern of all.”
I have the same question Ryan does — how do you do Parallax in the first movie? I could see it in maybe the second or third installment, but “Heroic star of the movie turns into villain” seems a little premature for the opening chapter. And now that I think about it, a Parallax / Fall of Hal Jordan movie would be great for Part 2, leading into a great Green Lantern III: Rebirth.