ThunderCats follows the adventures of cat-like aliens on a planet called Third Earth. The story begins with the dying planet Thundera, forcing the ThunderCats to flee their world. The fleet is attacked by their enemies, the Mutants of Plun-Darr, who destroy most of the fleet but spare the flagship hoping to find the mystic Sword of Omens on board. This sword holds the Eye of Thundera, the source of the ThunderCats’ power. Because of the damage to the ship, the ThunderCats must alter their destination and go on a very long journey to “Third Earth”. During this journey , Lion-O, as well as the ThunderCats Cheetara, Panthro, Tygra, WilyKit and WilyKat, and Snarf sleep in capsules.
When they awake on Third Earth after ten “galacto-years,” Lion-O discovers that his capsule has altered his aging process. He is now a child in the body of an adult. He is now a grown-up without maturity. On their new planet, the ThunderCats and the friendly natives of Third Earth build the “Cat’s Lair,” a new home and headquarters. Unfortunately, the Mutants have tracked them down. The demonic, mummified sorcerer Mumm-Ra recruits the Mutants to help him get the Eye of Thundera and destroy the ThunderCats in order to continue his evil reign over Third Earth.