On the planet Eternia, Skeletor and his dark army overthrow the Sorceress of Castle Grayskull expecting to acquire her power. He-Man, his old friend Duncan "Man-at-Arms" and his daughter Teela are attacked by Skeletor's soldiers and they defeat them. They also rescue their prisoner, the inventor and locksmith Gwildor. He explains that he was lured by Evil-Lyn that used his invention Cosmic Key to open the gates and seize the Castle Grayskull. He-Man and his friends retrieve the prototype of the Cosmic Key trying to release the Sorcereress but they are defeated by Skeletor and his army and Gwildor uses his key to open and portal for them to flee. They come to Earth but lose the key. Meanwhile Julie Winston, who grieves the loss of her parents in a plane crash, and her boyfriend Kevin Corrigan find and activate the key, believing it is a foreign musical instrument. On Eternia, Evil-Lyn locates the Cosmic Key and Skeletor sends her with a group of mercenaries and soldiers to vanquish He-man and his friends and bring the key back. Will they succeed?