Superman III (1983)
August "Gus" Gorman, a chronically unemployed ne'er-do-well, discovers a knack for computer programming. After embezzling from his new employer's payroll (through a technique known as salami slicing), Gorman is brought to the attention of the CEO Ross Webster. Webster is obsessed with the computer's potential to aid him in his schemes to rule the world financially, and is more impressed than angry at Gorman's embezzlement. Joined by his sister Vera and his "psychic nutritionist" Lorelei Ambrosia, Webster blackmails Gorman into helping him.
Meanwhile, Clark Kent has convinced his Daily Planet boss Perry White to allow him to return to Smallville for his high school reunion. En route, as Superman, he extinguishes a fire in a chemical plant containing vials of highly-unstable Beltric acid that can produce clouds of corrosive vapor when superheated. At the reunion Clark is reunited with childhood friend Lana Lang, a divorcée with a young son named Ricky.
Webster schemes to monopolize the world's coffee crop. Infuriated by Colombia's refusal to do business with him, he orders Gorman to command an American weather satellite named Vulcan to create a tornadic storm to decimate the nation's coffee crop. Webster's scheme is thwarted when Superman neutralizes the tornado and saves the harvest. Webster then orders Gorman to use his computer knowledge to create Kryptonite, remembering Lois Lane's Daily Planet interview with Superman, in which Superman identified it as his only weakness. Gus uses a computer to order Vulcan to locate Krypton's debris in outer space, but after the computer fails to analyze an "unknown" element in kryptonite, he improvises by replacing the unidentified element with tar, garnered from a pack of cigarettes.
Lana convinces Superman to appear at Ricky's birthday party, but Smallville turns it into a town celebration. Gus and Vera, disguised as United States Army officers, give Superman the Kryptonite as a gift, but are dismayed to see that it appears to have no effect on him. However, because this version of Kryptonite was not perfect, the compound gradually produces symptoms. Superman goes through a descent into darkness as he becomes selfish, focusing on his lust for Lana, which causes him to delay rescuing a truck driver from his jackknifed rig. Superman begins to question his own self-worth, he becomes depressed, angry, and casually destructive, committing petty acts of vandalism such as blowing out the Olympic Flame and straightening the Leaning Tower of Pisa.
Webster plots to control the world's oil supply, ordering Gorman to direct all tankers to the middle of the Atlantic Ocean and have them sit idly until further notice. Gorman feels unappreciated. He gives Webster a series of crudely drawn blueprints for a supercomputer. Ross makes a deal with Gorman, agreeing to build his supercomputer in return for sorting out the oil tankers.
Superman goes on a drinking binge, but is eventually overcome by guilt and undergoes a nervous breakdown, after Ricky calls out to him urging him to fight against his descent into evil. After nearly crash-landing in a junkyard, Superman splits into two personas: the immoral, selfish, corrupted Superman and the moral, righteous Clark Kent. They engage in a battle, ending when Clark strangles his evil identity, vanquishing him for good. Restored to his normal heroic self, Superman sets off to repair the damage his evil counterpart had caused.
After defending himself from numerous rockets and an MX missile en route to the Grand Canyon and the villains' hideout, Superman confronts Webster, Vera and Lorelei for a final showdown. He is forced into a battle with Gorman's supercomputer, which severely weakens him with a beam of pure Kryptonite. Gorman, guilt-ridden and horrified by the prospect of "going down in history as the man who killed Superman", destroys the Kryptonite ray with a firefighter's axe, whereupon Superman flees. The computer becomes self-aware and begins to defend itself against Gus's attempts to disable it, draining power from electrical towers, causing massive blackouts. Ross and Lorelei escape from the control room, but Vera is pulled into the computer and forcibly transformed into a cyborg. Empowered by the supercomputer, Vera attacks her brother and Lorelei with beams of energy that immobilize them. Superman returns to the battle with a canister of the Beltric acid from the chemical plant he saved earlier, the intense heat emitted by the machine causes the acid to turn volatile, eventually destroying the supercomputer. Superman flies away with Gus, putting him in a West Virginia coal mine, leaving Webster and his cronies to deal with the authorities.
Superman returns to Metropolis and reunites with Lana, who has relocated to the big city and found employment as the new secretary to Perry White. The film ends with Superman flying into the sunrise for further adventures.
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