The inhabitants of the planet Thundera evacuate just before it is destroyed but are pursued by a band of mutants. All but one of the escape ships is destroyed and a small group of Thunderans remain. With only half engine power, the group, led by Jaga, set a course for the nearest planet. Jaga commands the ship while the others are safely in their stasis tubes. Jaga dies during the journey to Third Earth and their ship crashes. After awaking, they make friends with the inhabitants in the area and then design a fortress. Mumm-Ra the centuries-old embodiment of evil, along with the mutants that destroyed the rest of the Thunderans continue to threaten them. Lion-O, the new leader of the Thundercats, with his weapon the Thundercats Sword "Sword of Omen" helps give the Thundercats a fighting chance to survive and eventually live in peace on the new home world.