A volcano bursts from out of nowhere, releasing the beast known as the Rock Giant. The Rock Giant is a powerful monster made entirely out of stone. Mumm-Ra decides to use the giant to destroy the Thundercats. While this is all happening a storm happens due to the erruption which has the left the Thunderkittens trapped in a forest.Lion-O and Panthro attempt to rescue them but before they can do so Jackalman attacks the thunder-kittens in one of the skycutters. The Thunderkittens are able to defeat Jackalman by using their magic pellets and summoning a flying saucer which causes Jackalman to grab ahold of the Skycutter. Thinking it's hostile, the Thundercats shoot the Skycutter and cause the Thunderkittens to barely be shot out of the sky.
The Thundercats leave in their Thundertank and attempt to arrive back at their lair, but are stopped by the Rock Giant who is pursuing the machine. Lion-O, Panthro, and the Thunderkittens are able to escape from the tank before the rock giant nearly destroys it. The Thundercats attempt to face the Rock Giant but find that their weapons aren't working. Eventually Lion-O is able to persue the beast to face him alone.
Durning the fight Jaga appears and tells Lion-O to remember how the ancient one used the power of heat to break even the hardest of rocks. Lion-O is able to use the Sword of Omens to push the giant to some white heat. He then lures him into the cold water near the lair's moat. The temperature destroys the Rock Giant.