An unforeseen betrayal by a former comrade brings destruction upon the Cat Kingdom. While King Claudus leads the fight against General Slithe and his lizard army, the evil sorcerer Mumm-Ra attempts to seize the Sword of Omens.

The ThunderCats accompany Panthro to Cloud Peak Mine to search for Thundrilium for the Thundertank. Along the way, Panthro and Grune's past is explored, from their first encounter to the reason behind Grune's betrayal.

The ThunderCats continue their quest for the Book of Omens, which leads them to the fabled Tower of Omens. However, they will first have to get past a series of deadly traps in order to reach it. Meanwhile, Mumm-Ra grows tired of waiting and forces Jaga to reveal the Book's location, and plans to go there himself to take it at all costs.

Lion-O ventures into the Book of Omens where he becomes the incarnation of his renowned ancestor, the valiant Leo, who led the rebellion against Mumm-Ra in a previous millennium.

The Cats are ready to retrieve the stone from the "Astral Plane" in the Elephant Village, but find it overtaken by Grunea and Mumm-Ra's forces. Plus, Tygra and Cheetara's early relationship to one another is revealed.

Tygra and Lion-O enter the Astral Plane to retrieve the stone, but encounter Mumm-Ra in the process. Grune and Panthro have their final duel. Cheetara finally chooses her beau.

The Thundercats require the assistance of a snake oil salesman named Ponzie to defeat Mumm-ra’s latest monstrous form.