The ThunderCats go on the offensive, but Mumm-Ra won't see his lizard army crushed so easily. He tasks Slithe with recruiting two new and vicious Generals.
During an ambush, Lion-O falls to his death, but he is given a second chance at life - if he can pass a series of trials that pit him against each of his teammates.
The ThunderCats must infiltrate the mines of Mount Plundarr where Ratar-O is holding Cats as slaves. They learn the legend of the Sword of Plundarr, which they must retrieve.
As Lion-O and Pumyra try to evade Mumm-Ra and his forces in the mines of Mount Plundarr in an effort to keep the Sword of Plundarr out his hands, we flash back in time and discover its origin ... and its relationship with the Sword of Omens.
When Mumm-Ra unleashes the deadly Sycorax, the ThunderCats turn to an unlikely savior -- a snake oil salesman named Ponzi whose potion is the only thing that repels the beast.
While trying to uncover the location of the next stone, the ThunderCats cross paths with a mad man, Soul Sever, who is trying to insert the souls of his long-lost family into machines.
The ThunderCats' search for the Tech Stone leads them high above Thundera to the floating bird city of Avista, where the stone is the power source for their city.
In the season finale, Mumm-Ra's forces find the ThunderCats and the Tech Stone in Avista City. An incredible battle ensues for the stone as Lion-O finds himself betrayed by a trusted ally.