Masters of the Universe

On the planet Eternia, the warlord Skeletor has conquered Castle Grayskull and taken the Sorceress prisoner, slowly draining her life force. He awaits the alignment of Eternia's moon with the Great Eye of the Universe, an event that will grant him god-like, cosmic power. He-Man, along with veteran soldier Man-At-Arms and his daughter Teela, rescues Gwildor, a diminutive Thenorian locksmith who invented the Cosmic Key — a device that opens dimensional portals using sequences of musical tones. Skeletor stole Gwildor's original Key, but Gwildor possesses a second prototype. When He-Man's group attempts to free the Sorceress from Grayskull, they are vastly outnumbered and forced to flee through a hastily opened portal. They arrive on present-day Earth in a small California town, but the Cosmic Key is lost in the transit. The Key is found by two local teenagers: Julie Winston, an orphaned high school student haunted by the recent death of her parents in a plane crash, and her boyfriend Kevin Corrigan, an aspiring musician. Kevin mistakes the device for an exotic synthesizer and takes it to a music store where he works. While they experiment with it, the Key emits a signal that Evil-Lyn, Skeletor's sorceress lieutenant, uses to pinpoint its location on Earth. She dispatches a squad of Skeletor's mercenary warriors — Saurod, Blade, Beast Man, and Karg — to retrieve it. He-Man intercepts the henchmen and rescues Julie. A suspicious local detective, Hugh Lubic, gets drawn into the chaos and confiscates the Key. Evil-Lyn captures Kevin and uses a mind-control collar to force him to help her track down Lubic and recover the device. A ferocious battle erupts at the music store; Julie is wounded in the fighting. Evil-Lyn ultimately recovers the Key and uses it to summon Skeletor himself to Earth. He-Man is captured, bound, and transported back to Eternia as a slave to be displayed before Skeletor's triumph. On Earth, Gwildor repairs the second prototype Key, and Kevin — drawing on his musical ear — recreates the correct tonal sequence to open a new gateway back to Eternia. He-Man's allies, Kevin, Julie, and an unwilling Lubic are all pulled through the portal to Castle Grayskull. As the moon reaches its apex, the Great Eye of the Universe opens and Skeletor absorbs the full power of the cosmos. His armor transforms from black to gleaming gold and his hood is replaced by a towering helmet fashioned in Grayskull's likeness; he declares himself the Master of the Universe and proclaims He-Man no longer his equal. Resentful that Skeletor claimed all the power for himself, Evil-Lyn orders Karg to sound the retreat, abandoning Skeletor. He-Man breaks free of his bonds, retrieves the Sword of Grayskull, and engages Skeletor in a sword duel. He shatters Skeletor's staff, stripping away the cosmic power and reverting Skeletor to his normal state. He-Man offers mercy, but Skeletor draws a concealed blade and lunges to kill him. He-Man dodges and Skeletor plunges into a deep pit below the castle. The Sorceress is freed and Castle Grayskull is restored. In a parallel resolution, Julie discovers she has been shifted slightly back in time; she uses the opportunity to call and warn her parents not to board the fatal flight, saving their lives. Lubic, now stranded on Eternia, decides to stay and build a new life there. In a post-credits scene, Skeletor's hand breaks the surface of the pit's water, revealing he survived.

Sources: Wikipedia, Web search (aggregated)