The ThunderCats team up to combat a new foe: Skeletor! And as young King Lion-O battles this evil from another world, he distracts Skeletor from an important fight against Mumm-Ra. With the Power of Grayskull hanging in the balance, who can possibly turn the tide of battle?
The Thundercats fight Skeletor to get back He-Man's Sword of Power. The tides keep changing until Lion-O realizes the Skeletor has merged with Mumm-Ra. Lion-O then shows Skeletor his own reflection and this makes Skeletor/Mumm-Ra run away. Skeletor/Mumm-Ra realize they need to work together and come up with an evil plan.
Lion-O and the Thundercats take the Sword of Power back to Eternia to give it back to is true ower, He-Man, but He-Man is dead. Lion-O then tries to give the sword to King Randor but is interrupted when the castle is attacked by the Spirit of Evil, being controlled by Skeletor/Mumm-Ra.