The epic series comes to the only conclusion possible: all-out war against Mumm-Ra and Skeletor! He-Man and Lion-O confront the diabolical duo in the heart of the magical, mystical Castle Grayskull, but the battle royal quickly expands across the multiverse itself!
He-Man, Lion-O and Battle Cat return to Eterina from Third Earth, to find the Masters & Thundercats defeated and the gates to Castle Grayskull wide open. He-Man and Lion-O gather their courage and head inside. There they find Mumm-ator with the Orb of Power in the Nexus of all Realities, holding the Sorceress captive.
The pair of heroes attack the combine villain but are pulled through realities by Mumm-ator. They fight through realities where Mummra rules, a Earth where He-Man lives among Earthlings, a Thundercat Eterina, and a Thundera before it's destruction. They linger on the last one so Lion-O can see his father one last time, but He-Man pulls them back the present and current reality.
The Sorceress has escaped and has used her magic to heal the Masters and Thundercats and directs their combined forces to fight Mumm-ators combined force of Evil and Mutants. He-Man and Lion-O uses their combine power to destroy Mumm-ator.
Later, the Masters say goodbye to their Thundercat counter parts, as Third Earth returns to its time and space.