Superman III

Gus Gorman, a chronically unemployed loser, discovers he has a talent for computer programming and gets a job at the Metropolis-based conglomerate Webscoe. Gus, dissatisfied with his pay, embezzles from his employer using a technique known as salami slicing, which brings Gus to the attention of CEO Ross Webster. Webster is intrigued by the computer's potential to aid him in his schemes to rule the world financially. Joined by his sister Vera and his "psychic nutritionist," Lorelei Ambrosia, Webster blackmails Gus into helping him.
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 highly unstable Beltric acid that can produce corrosive vapor when superheated. At the reunion, Clark is reunited with childhood friend Lana Lang, a divorcÃ©e with a young son named Ricky, and harassed by Brad Wilson, her ex-boyfriend.
Webster schemes to monopolize the world's coffee crop. Infuriated by Colombia's refusal to do business with him, he orders Gus to command a weather satellite, Vulcan, to create a tornado to destroy Colombia's coffee crop for the next several years. Gus travels to Smallville to use the offices of WheatKing, a subsidiary of Webscoe that manufactures farm equipment, to reprogram the satellite. Though Gus is successful and Vulcan creates a devastating storm, Webster's scheme is thwarted when Superman flies into the eye of the storm, neutralizing it and saving the harvest. Webster, seeing Superman as a legitimate threat, orders Gorman to use his computer knowledge to create Kryptonite, remembering Lois Lane's Daily Planet interview with Superman. Gus uses Vulcan to locate Krypton's debris in outer space and to analyze its components; he discovers that one of the elements of Kryptonite is an "unknown" compound, but (glancing at his pack of cigarettes) decides to substitute tar instead, thinking no one will notice.
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 crude Kryptonite as a gift, but it appears to cause no symptoms; however, Superman is seen biting his nails as he studies the kryptonite. Superman soon becomes selfish, focusing on his lust for Lana, which causes him to delay rescuing a truck driver from his jackknifed rig. Superman begins questioning his self-worth and 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.
Gus, feeling used, gives Webster crude plans for a supercomputer and Webster agrees to build it in turn for Gus directing all oil tankers to sail the middle of the Atlantic Ocean and remain there, idle, until further notice. When the captain of one tanker insists on maintaining his course for Metropolis, Webster uses Lorelei to seduce Superman into waylaying the tanker, breaching its hull and causing a massive oil spill. The villains decamp to the computer's location in Glen Canyon.
Superman goes on a drinking binge, but is overcome by guilt and undergoes a nervous breakdown. After nearly crash-landing in a junkyard, Superman splits into two personas: the immoral, corrupted Superman and the moral, righteous Clark Kent. They engage in a battle, ending when Clark strangles his evil identity. Restored to his normal self, Superman repairs the damage his evil counterpart caused.
After defending himself from rockets and an MX missile, Superman confronts Webster, Vera, and Lorelei. Gus's supercomputer immediately identifies him as a threat and attempts to find his weakness. After a beam of energy and a constricting plastic bubble fail to stop Superman, the computer finally succeeds with a beam of pure Kryptonite.
Guilt-ridden and horrified by the prospect of "going down in history as the man who killed Superman", Gus destroys the Kryptonite ray with a firefighter's axe, whereupon Superman flees. The computer becomes self-aware and defends itself against Gus's attempts to disable it, draining power from electrical towers. Ross and Lorelei escape from the control room, but Vera is pulled into the computer and transformed into a cyborg. Vera attacks her brother and Lorelei with beams of energy that immobilize them. Superman returns with a canister of the Beltric acid from the chemical plant he saved earlier. Superman places the canister by the supercomputer, which does not resist as it suspects no immediate danger. The intense heat emitted by the supercomputer causes the acid to turn volatile, destroying it. Superman flies away with Gus, leaving Webster and his cronies to deal with the authorities, and drops Gus off at a West Virginia coal mine.
Superman returns to Metropolis. As Clark, he pays a visit to Lana, who has relocated to the big city and found employment as Perry White's new secretary. He is attacked by Brad, who has stalked Lana to Metropolis, only to end up falling into a room service cart. The film ends with Superman flying into the sunrise for further adventures.

Question: What does Clark Kent come back in Smallville for?
Answer: his high school reunion