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 discover that they have to find three stones of power before Mumm-Ra does. A fourth stone of power is revealed by the book near the end of the first season to be the Eye of Thundera itself, which is embedded in the Sword of Omens.