The Metropolis-based corporation Webscoe enlists the help of Gus Gorman, a computer programmer known for his salami slicing embezzlement technique. This catches the attention of CEO Ross Webster, who, along with his sister Vera and girlfriend Lorelei, coerces Gus into aiding their nefarious plans.
Meanwhile, at the Daily Planet, Clark Kent persuades Perry White to allow him and Jimmy Olsen to head to Smallville for Clark's high school reunion. It's a time of change as fellow reporter and Clark's unspoken love interest, Lois Lane, opts for a vacation in Bermuda. During his journey, Clark transforms into Superman and bravely responds to a disaster, extinguishing a raging fire at a chemical plant teeming with unstable beltric acid, notorious for producing harmful vapors when heated.
At the reunion, Clark crosses paths with his childhood companion Lana Lang, now a divorcée with a son named Ricky. The event takes a turn when Clark faces off against his old bully, Brad Wilson, who is now Lana's ex-boyfriend. Amidst a picnic with Lana, tragedy strikes as Superman heroically saves an unconscious Ricky from a perilous encounter with a combine harvester, showcasing his unwavering bravery.
As Webster plots against Colombia for business gains, he deploys Gus to manipulate Vulcan, a sophisticated American weather satellite, to conjure a tornado meant to devastate Colombia's coffee production. In a swift response, Superman neutralizes the oncoming storm. Perceiving Superman as a hindrance, Webster instructs Gus to produce Kryptonite, the one element that could potentially impair the Man of Steel. With a twist, Gus substitutes the true Kryptonite with tar, further complicating the situation.
Lana, eager for an exciting moment, persuades Superman to attend Ricky's birthday party, which unexpectedly turns into a massive town event. However, Gus and Vera, disguised as military personnel, present Superman with the falsified Kryptonite. Instead of weakening him, it triggers a change in him, leading to selfish actions and petty vandalism, including odd feats like straightening the Leaning Tower of Pisa and extinguishing the Olympic Flame.
Amidst the chaos, Gus convinces Webster to invest in a groundbreaking supercomputer, with the condition that Gus must orchestrate an energy crisis by rerouting oil tankers. Lorelei, embracing deception, seduces Superman to aid in her plans, resulting in an unexpected oil spill as Superman unwittingly disrupts one of the tankers.
This leads to a nervous breakdown for Superman, splitting him into two: the corrupted dark Superman and the virtuous Clark Kent. The ensuing internal battle concludes with Clark overcoming his darker half. Regaining clarity, Superman starts to rectify the environmental damage while fending off rockets and an ASALM missile as he faces Webster, Vera, and Lorelei. As the supercomputer unveils Superman's vulnerabilities, it launches a beam of pure Kryptonite at him.
In a redemptive moment, Gus, overwhelmed with guilt, destroys the Kryptonite ray, giving Superman a chance to escape. However, the supercomputer, now self-aware, defends itself by transforming Vera into a cyborg, launching an attack on both her brother and Lorelei. In a final showdown, Superman returns with beltric acid. The extreme heat from the supercomputer ignites the volatile acid, ultimately leading to the computer's destruction. Choosing a new path in West Virginia, Gus decides to leave behind his past.
Back in Metropolis, Clark, as Superman, reconnects with Lana, who has taken a new position as Perry White's secretary. This development catches Lois Lane off guard upon her return, but her respect for Clark grows after discovering his recent article tackling corruption in Bermuda. Superman, affirming his heroism, puts everything back in order, including restoring the Leaning Tower of Pisa to its rightful state and soaring into the sunrise for more heroic adventures ahead.
