Millions of years before the Earth was formed, a group of beings called the Guardians of the Universe used the green essence of willpower to create an intergalactic police force called the Green Lantern Corps. They split the universe into 3,600 sectors, with one Green Lantern per sector. One such Green Lantern, Abin Sur of Sector 2814, defeated the being Parallax and imprisoned him in the Lost Sector on the ruined planet Ryut. In the present day, Parallax escapes from his prison. Six months later, after killing four Green Lanterns and destroying three planets, Parallax attacks Sector 2814 and mortally wounds Abin Sur, who escapes and crash-lands on Earth. The dying Abin Sur commands his ring to find a worthy successor on the planet.
A cocky Ferris Aircraft test pilot Hal Jordan is chosen by the ring and transported to the crash site, where Abin Sur appoints him a Green Lantern, by telling him to take the lantern and speak the oath. At home he says the oath of the Green Lanterns while under trance from the glow of the lantern. After he is attacked by employees laid off from Ferris Aircraft while leaving a bar, Jordan throws a punch at one of his attackers, but instead of connecting with his fist, the ring creates a huge fist of green energy. Jordan is whisked away to the Green Lantern Corps home planet of Oa, where he meets and trains with Tomar-Re and Kilowog. He encounters Corps leader Sinestro, who is displeased that a human—a primitive compared to other Corps species—has become a Green Lantern. With Sinestro believing him to be unfit and fearful, Jordan quits and returns to Earth, keeping the power ring and lantern.
Meanwhile, after being summoned by his father Senator Robert Hammond to a secret government facility, scientist Hector Hammond performs an autopsy on Abin Sur’s body. A piece of Parallax inside the corpse inserts itself inside Hammond, mutating the scientist and giving him telepathic and telekinetic powers at the cost of his sanity. After discovering that he was only chosen due to his father’s influence, Hammond resentfully attempts to kill his father by telekinetically sabotaging his helicopter at a party. Jordan uses his ring to save the senator and the party guests, including his childhood sweetheart Carol Ferris, a fellow test pilot and manager of Ferris Aviation. She recognizes Jordan despite his costume and mask. Shortly afterward, Hammond successfully kills his father by burning him alive despite Jordan’s interference. Both Jordan and Hammond realize Parallax is on his way to Earth.
Back on Oa, the Guardians tell Sinestro that Parallax was once one of their own until he desired to control the yellow essence of fear, only to become the embodiment of fear itself. Believing the only means to fight fear is by fear itself, Sinestro requests that the Guardians to forge a ring of the same yellow power, preparing to concede Earth’s destruction to Parallax in order to protect Oa. However, Jordan appears and tells Sinestro not to use the yellow ring and requests the Corps’s assistance in protecting Earth from Parallax’s imminent invasion. They deny his request but allow Jordan to return to his home planet.
Upon returning to Earth, Jordan saves Ferris from being injected with Parallax’s essence by Hammond. Parallax arrives, consuming Hector’s life force, and then wreaking havoc on Coast City. Jordan is nearly killed during his battle with Parallax, but realizes why he was chosen by the ring: true bravery is not being fearless, it is the ability to overcome fear. With new-found strength, Jordan lures Parallax away from Earth and toward the Sun, using the Sun’s gravitational pull to disintegrate the entity. He loses consciousness after the battle, but is saved by Sinestro, Kilowog, and Tomar-Re. Later the entire Green Lantern Corps congratulates him for his bravery. Sinestro tells Jordan he now bears the responsibility of protecting his sector as a Green Lantern.
In a post-credits scene, Sinestro steals the yellow ring and places it on his finger, causing his green suit and eyes to change to yellow.
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