Thundera is attacked by lizards who have acquired salvaged ancient technology. In face of betrayals, revelations and tragedies, Lion-O and Tygra mount a rescue with the team.
After escaping their destroyed city, the team encounters the Fishmen. The ThunderCats' quest is to find the Book of Omens ... but Lion-O is more interested in revenge.
While hiding from the unstoppable lizard army, Lion-O encounters a race of creatures who live out their entire lifetime in 24 hours, and learns a valuable lesson about hope.
Panthro finally joins the team, but his gruff demeanor intimidates Lion-O. Despite his best efforts, Panthro can't dissuade the Cats from following him on a mission to get Thundrillium to fuel the Thundertank.
Lion-O ventures into the Book of Omens where he becomes the incarnation of his renowned ancestor, the valiant Leo, who led the rebellion against Mumm-Ra in a previous millennium.
While traveling, Lion-O's cockiness loses him the Sword of Omens to the Duelist, a villainous sword-collector. With the help of the listless Drifter, Lion-O must get it back.
The Cats track the first stone's location to the mystical Elephant Village, but the forgetful Elephants can't remember where it is.Lion-O must learn to use Sight Beyond Sight.
Sent by the Elephants to the Forest of Magi Oar, Lion-O must learn to master Sight Beyond Sight before returning to the Elephant Village. The Cats encounter a group of paper warriors with a dark secret.
The Cats are ready to retrieve the stone from the "Astral Plane" in the Elephant Village, but find it overtaken by Grunea and Mumm-Ra's forces. Plus, Tygra and Cheetara's early relationship to one another is revealed.
Tygra and Lion-O enter the Astral Plane to retrieve the stone, but encounter Mumm-Ra in the process. Grune and Panthro have their final duel. Cheetara finally chooses her beau.
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.
