On the planet Eternia, the dark lord Skeletor has conquered the mystical fortress Castle Grayskull and imprisoned the Sorceress. His goal is to harness Grayskull's power to become the master of the universe. However, the heroic warrior He-Man, alongside his loyal friends — the inventor and locksmith Gwildor, the brave soldier Man-At-Arms, and his daughter Teela — refuse to concede defeat. Gwildor reveals a device called the Cosmic Key, capable of opening portals to any point in time and space. With Skeletor's forces in hot pursuit, they resolve to launch a daring assault on Castle Grayskull to free the Sorceress and put an end to Skeletor's tyranny.

During a skirmish with Skeletor's minions, the Cosmic Key is activated, opening a portal that accidentally transports He-Man and his allies to Earth. Stranded on our planet, they encounter a couple of teenagers, Julie and Kevin, who become unwittingly entangled in the battle. The Key is lost amidst the chaos of arrival, necessitating a desperate search as Skeletor sends a band of mercenaries to Earth to reclaim it. As the race against time unfolds, both groups vie for control of the Key — He-Man and his friends in the hope of returning to Eternia to stop Skeletor, and the dark lord's followers to seize it for the completion of their overlord's scheme.