Formulaic, bland, and improbable, the film's greatest flaw lies in its shallow moral message
Billions of years ago, we are informed in the preamble of Green Lantern—a preamble in which the portentous and expository vie for supremacy before settling for a draw—a race of immortals called the Guardians harnessed the most powerful energy in the universe, a green force generated by every living thing. This force, with apologies to The Merchant of Venice, represents not jealousy but "the will"—and, yes, if the historical echoes are somewhat unfortunate, be forewarned that they will recur. (Note to filmmakers: Having a promotional poster in which the martial heroes are all raising one fist in salute really doesn't help.)
Where was I? Right, in space. All was well until an evil, countervailing entity called Parallax arose, powered by a fear-energy colored yellow (or was that "yella?"), and voiced by Clancy Brown—who ought to have this schtick down, having over the years provided the menacing intonations of, among other legendary baddies, Hades, Lex Luthor, and Eugene H. Krabs of SpongeBob fame. Fortunately, Parallax is captured and imprisoned on the planet Ryut by a powerful "Green Lantern," one of a corps of 3,600 cosmic policemen empowered by the Guardians. Less fortunately, his prison is evidently devoid of guards, alarms, or structural integrity. When a few harmless aliens set foot on the planet, they immediately fall through the crust into the chamber where he's held, thus ending the era of quiet Ryut. Parallax escapes, looking something like a cross between Voldemort and the flying fetus from Hellraiser. After devouring a few planets and packing on a couple million pounds, though, he comes to resemble the intergalactic offspring of Cthulhu and a crude-oil spill.
Cut to present-day Earth, where test-pilot Hal Jordan (gifted? check; impulsive? check) is test-dogfighting a couple of high-tech drone jets, which he ultimately defeats by risking his life (check) and crashing his plane (check). Various muckety-mucks are displeased with Hal (played by Ryan Reynolds), as is his flying-partner/sometimes-girlfriend/CEO-in-training-of-the-aeronautics-company Carol Ferris (Blake Lively). To make matters worse, Hal's near-death antics have spoiled the 11th birthday party of a beloved nephew, who appears in this one scene and is then, with the rest of Hal's family, completely forgotten. Oh, and then an alien Green Lantern, mortally wounded by Parallax, crash lands on Earth, and his ring—from which the Lanterns get their power; I mentioned this didn't I?—chooses Hal's finger as its new home.
Familiar motions are gone through: the experimentation with great powers (flight, the ability to create a green force field in the shape of anything Hal imagines), the rejection of same when he discovers that with them comes great responsibility, etc. Hal is briefly whisked away for training on Oa, the planet of the Green Lantern Corps, but when the other Lanterns convince him that he's unworthy and the ring made a "mistake," he quits. (Yes, this sequence is essentially Kung Fu Panda, without the panda or the kung fu.)
Reynolds's and Sarsgaard's characters are like high-school stereotypes inflated to Nietzschean proportions.
Back on Earth, a homely xenobiologist named Hector (Peter Sarsgaard), who happens to be the exquisitely improbable son of a John-Ensign-pompadoured Senator (Tim Robbins), has been infected by a piece of Parallax, and begins discovering powers of his own: telepathy, telekinesis, and a tendency toward grotesque head-bloat. Hal returns from Oa to quarrel with an increasingly dangerous Hector, rescue the Senator, rescue Carol, come to grips with his fears, and fulfill other narrative obligations that you can probably guess more quickly than I can type them. Parallax decides that Earth and its billions of souls are next on his menu of interstellar entrees, leading to a finale in which—never mind, I'm sure you're ahead of me already.