En Sabah Nur, a powerful mutant believed to be the first of his kind, rules prehistoric Egypt until he is betrayed by his worshippers, who entomb him alive. His four lieutenants die preserving him. Awakening in 1983, he believes humanity has lost its way without his presence. Aiming to destroy and remake the world, he recruits Cairo pickpocket Ororo Munroe, who can control the weather, and upgrades her power.
In East Berlin, shape-shifting mutant Raven investigates an underground fight club and discovers mutant champion Angel, who possesses a pair of large feathered wings on his back, and Kurt Wagner, who can teleport. Raven rescues Kurt and employs the services of black marketeer Caliban to transport him to America. En Sabah Nur recruits Caliban’s enforcer, Psylocke, who leads him to Angel. En Sabah Nur enhances both their powers, transforming Angel’s wings into metal.
Alex Summers discovers that his younger brother Scott is manifesting a mutation for shooting optic beams. Alex takes Scott to Professor Charles Xavier’s educational institute in Westchester County, New York, hoping that Xavier and Hank McCoy will teach him to control his abilities. Scott meets the telepathic and telekinetic Jean Grey, and the two develop an attraction. Raven brings Kurt to the institute. En Sabah Nur’s powers cause disturbances around the world, leading Xavier and Alex to consult with CIA agent Moira MacTaggert, who has been researching the legend of Nur.
In Communist Poland, the metal-controlling mutant Erik Lehnsherr lives with his wife and their young daughter, Nina. He uses his powers to save a coworker, prompting the Militia to capture him. When they accidentally kill Erik’s family, he retaliates by executing them. En Sabah Nur later approaches the devastated Erik and takes him to Auschwitz, where Erik’s power first manifested. Erik destroys the camp and joins him.
En Sabah Nur remotely accesses Cerebro, a brainwave-amplifying device Xavier uses to locate mutants. Through it, he forces the telepathic Xavier to make the global superpowers launch their entire nuclear arsenals into space to prevent interference with En Sabah Nur’s plan. He and his new lieutenants arrive at the mansion and kidnap Xavier. Attempting to stop them, Alex accidentally causes an explosion that destroys the mansion. Peter Maximoff, having learned that he is Erik’s son and hoping that Xavier can help to find him, arrives just in time; he uses his super-speed to evacuate the students before the explosion destroys the building, but Alex is presumed dead. Colonel William Stryker’s forces subsequently capture Hank, Raven, Peter, and Moira, and take them to a military facility for interrogation. Scott, Jean and Kurt follow covertly, and liberate their comrades using Stryker’s mind-controlled and brainwashed experiment Weapon X, whose memories Jean partially restores.