Vultureman escapes from exile by hijacking a bookmobile. He finds a disk containing ancient history about two powerful evil forces on Third Earth: Mumm-Ra and his enemy Zaxx.
Zaxx had used a Rosencrest medallion, but Zaxx was defeated when Mumm-Ra stole the medallion trapping Zaxx in it. Vultureman plots to get the medallion, defeat Mumm-Ra, and rule the universe.
Meanwhile, Zaxx uses Vultureman to find and defeat Mumm-Ra. But there's a terrible "side effect" to the medallion, anyone who wears it loses his own life and BECOMES Zaxx. When Vultureman learns of this, he changes his mind.
The Thundercats, who were looking for Vultureman based on Mandora's warning that he was headed to Thundera, get caught in this situation when the mutant attacks Panthro and Snarf. After they crash, Snarf disappears.
Zaxx then uses Snarf to attack Mumm-Ra and Zaxx takes over Snarf's body. The other Thundercats arrive and save Snarf. Mumm-Ra defeats Zaxx once more by taking the medallion, but the pyramid also collapses on Mumm-Ra.