Vultureman invents the Shifter, a device that can switch personalities, and uses it on Panthro's personality and Snarf's body.

While trying to figure out a way to read the Book of Omens, Lion-O tries looking through the sword at the book. He is immediately drawn into the book, where a voice asks him if he has ""the key."" Lion-O says he doesn't, and four dragon heads appear and shoot fire at him. Snarf, who saw Lion-O get sucked into the book, tries to show Cheetara what happened, and he too gets sucked in. Mumm-Ra, who also watched Lion-O get sucked in, goes to cats lair and takes the book so he can destroy it with Lion-O inside. Meanwhile, just by coincidence, Panthro finds an odd looking key on Thundera. He and Tygra head back to Third Earth with the key, not knowing what it's for. Back on Third Earth, Mumm-Ra is just about to boil the book when Lion-O calls for the sword, and it brings him and Snarf back outside. Mumm-Ra takes the sword and looks through it at the book, and he gets sucked in! But Lion-O has to save him because of the code of Thundera, and Lion-O does save him. Panthro and Tygra arrive with t

A convergence of the Moons of Plun-Darr causes a drought on Thundera. Panthro and Snarf go looking for water, then Ezuka, the flying Serpent of Dreary Canal wanting to get water itself, attacks Panthro and Snarf and causes them to fall into the Swamp of the Laughing Lilypads, making them laugh out of control. Ezuka attacks the Lair, still looking for water. Lion-O uses the Sword to make some rain, then he and Ben-Gali use the Thunderstrike to melt some ice, but Mumm-Ra comes along and causes a flood. Lion-O calls for the others, which also free Panthro and Snarf from the laughter and they use some of the Swamps water to cause Mumm-Ra to laugh uncontrolably, and he falls in the flood waters and is washed away.

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 used a Rosencrest medallion, but Zaxx was defeated when Mumm-Ra stole the medalion trapping Zaxx in it. Vultureman plots to get the medalion, defeat Mumm-Ra, and rule the universe. So Zaxx uses Vultureman to find and defeat Mumm-Ra. But there's a terrible ""side effect"" to the medalion, anyone who wears it loses his own life and BECOMES Zaxx. When Vultureman learns of this, he changes his mind. The Thundercats get caught in this situation when Vultureman attacks Panthro and Snarf and after they crash, Snarf dissappears. 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 takin the medalion, but the pyramid also collapses on Mumm-Ra.