On Eternia, Skeletor's scientist Modulok has created a Gate-Maker machine that can transport entire cities to other planets, and Skeletor plans to use it to transport Eternos to Etheria. But Modulok is sick of Skeletor's disrespectful treatment of him, so he betrays him and takes the machine to Etheria to give it to Hordak. Skeletor angrily follows Modulok to Etheria, where Hordak has recruited Modulok as his scientist and is planning to use the Gate-Maker to invade Eternia. As long-time enemies Skeletor and Hordak battle for control of the Gate-Maker, it is up to both She-Ra and He-Man to stop the villains before they invade Eternia!