From beyond any known galaxy, bringing with them the laws and ideals of their doom planet Thundera, come the Thundercats. Together they are all sworn to serve their young lord, Lion-O and to instruct him in the secrets of the Eye of Thundera, embedded in the hilt of the Sword of Omens and the source of their power. Pursuing the Thundercats to third Earth, and determined to posses the Eye of Thundera for their own evil purposes, are the mutants of the planet Plun-Darr, lead by the evil Reptillian, Slithe who form an unholy alliance with the ancient devil priest, Mumm-Ra.