For generations, the Thunderians have lived and thrived in the kingdom of Thundera. However, one night the kingdom is attacked by the Lizard army, led by the evil sorcerer Mumm-Ra and Grune the Destroyer. After Thundera is destroyed, a small band of surviving Thunderians led by the young heir to the throne Lion-O (who wields the powerful Sword of Omens) flees in order to seek out the Book of Omens, which is said to have the knowledge needed to defeat Mumm-Ra. Even when they find it, they must keep the Book of Omens from falling into the wrong hands and find a way to defeat Mumm-Ra once and for all. After finding the book, the Thundercats have to find the three spirit stones before Mumm-Ra does.