The Thundercats discover a cat refugee, Pumyra, being forced to fight as a gladiator in a city of Dogs.

Arriving at Mount Plun-Darr, the ThunderCats and Pumyra discover that the Thunderian slaves are being forced to excavate the Sword of Plun-Darr for Ratar-O and his Rats.

Mumm-Ra arrives at Mount Plun-Darr and battles the Thundercats in an attempt to regain the Sword of Plun-Darr.

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.