Unlike the previous three movies, which were produced by Alexander and Ilya Salkind, the fourth movie was produced by Golan-Globus's Cannon Films, in association with Warner Bros.
Clark Kent / Superman learns that the United States and the Soviet Union may soon engage in nuclear war, threatening the survival of the planet. Before taking action, he departs to the North Pole to seek advice from the spirits of his Kryptonian ancestors at the Fortress of Solitude, who warn him to not interfere, but rather flee to other worlds where war is long forgotten. At a meeting of the United Nations, he tells the assembly that he is going to rid the Earth of all nuclear weapons. Over the next several days, Superman takes all the nuclear weapons and gathers them into a gigantic net in orbit above the planet. When he has almost all the weapons, he closes the net and tosses it into the sun.
Meanwhile, Lex Luthor's nephew Lenny helps break Luthor out of prison. The pair steal a strand of hair that Superman had donated to a museum, whereupon Luthor creates a genetic matrix from the strand of hair, and attaches it to the final American nuclear missile. After the missile is fired off into the air, Superman grabs the missile and throws it into the sun. A few moments after the missile explodes on the sun's surface, a ball of energy is discharged from the sun, which rapidly develops into a "Nuclear Man" (Mark Pillow). This Nuclear Man then finds his way to his "father" Luthor, who establishes that while he is indeed powerful, he will deactivate if isolated from the sun's rays or suitably bright artificial light. This resembles the Greek legend of Anateus, a wrestler whose strength came solely from his bodily contact with the Earth.
A worldwide battle soon follows between Luthor's creation and the Man of Steel. While successfully saving the Statue of Liberty, Superman is injured by the radioactive nails of Nuclear Man. To Lois' disgust, The Daily Planet, which has been turned into a tabloid newspaper, blares the headline that Superman is dead. Felled by radiation sickness, Clark staggers weakly to the terrace of his apartment, where he retrieves the last benevolent Kryptonian crystal and uses it to heal himself.
Nuclear Man develops a crush on Lacy Warfield, daughter of the tycoon who has bought The Daily Planet, and threatens mayhem if he is not introduced to her. Superman therefore agrees to take Nuclear Man to Lacy. In an attempt to disable the villain, Superman lures Nuclear Man into an elevator in the building, traps Nuclear Man in it, and pulls the elevator out of the building and flies to the dark side of the moon, heaving the elevator onto the ground there, not realizing that its doors are slightly open. As the sun rises, Nuclear Man breaks out of his makeshift prison and the two resume battle on the moon's surface. At the end of the battle, Superman is driven into the ground by his opponent.
Nuclear Man then returns to Earth, abducting Lacy and flying her into outer space (where she, strangely enough, is unaffected by the lack of breathable atmosphere and air pressure). Meanwhile, Superman pushes the moon out of its normal orbit, casting Earth into a solar eclipse which terminates Nuclear Man's powers. He then rescues Lacy from the arms of Nuclear Man, of whom he disposes by returning him to Earth and sealing him into the core of a nuclear power plant. Superman thwarts Lex and Lenny, returning Lex Luthor to prison while Lenny is taken to Boys Town.
Later, in a press conference, Superman declares only partial victory in his peace campaign, stating that "There will be peace when the people of the world want it so badly, that their governments will have no choice but to give it to them". He then flies into space, smiling optimistically.