Fleeing through space to escape their dying home world, the ThunderCats were attacked en route by their mortal enemies, the Mutants of Plun-Darr. After diverting their damaged flagship to a planet called Third Earth, the surviving ThunderCats now strive to rebuild their society in harmony with the new world's natives. But the Mutants, determined to possess the ThunderCats' mystical gem, the Eye of Thundera, have tracked them down - and they've also forged an alliance with Mumm-Ra, the devil-priest of Third Earth!

Now the ThunderCats, led by an inexperienced Lion-O, must band together as never before to protect their legacy from this combined enemy. But will they be able to withstand the onslaught of Mumm-Ra the Ever-Living, once he sets his eyes upon them?