The series pilot begins with the dying Thundera meeting its end, forcing the ThunderCats (a sort of Thunderean nobility) to flee their homeworld. The fleet is attacked by the Thundereans's enemies, the Mutants of Plun-Darr, who destroy all the starships in the "ThunderFleet," but spare the flagship hoping to capture the legendary Sword of Omens they believe is onboard. The sword holds the mystical Eye of Thundera, the source of the ThunderCats's power, embedded in the hilt...
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You can buy Thundercats: The Complete Seasons 1 and 2 on Amazon