When a meteorite made of pure Thundrillium crashes on Hook Mountain, the Thundercats decide to use it to replenish their depleted energy supply. But Lion-O must first gain the trust of the fierce Snowman.

Mumm-Ra tricks Lion-O into using the Sword of Omens against Tygra, causing the blade to break in half and lose its powers. The only way Lion-O can restore the Sword is to venture into a volcano and have the Inflamer reforge it. But the Inflamer requires a payment of gold... Has Lion-O taken on more than he can handle?

The Thundercats briefly show Lion-O how the time capsule they recovered from the caveman works. Curious to see more, Lion-O activates it during his night-time watch, and gets caught in its projection rays. The rays send him back in time to Thundera, just before the final tremor that destroyed the planet. Meanwhile, the Mutants have built an evil robot monster that is destroying Cats' Lair. Back on Thundera, Lion-O encounters his father, Claudus, and saves him from a Mutant attack. In return, Claudus gives Lion-O the plans to the robot that was attacking the Thundercats in his own time.

Wiz-Ra, Mumm-Ra's enemy from long ago, comes back every 7000 years through a portal that just happens to be in Cheetara's bedroom. Wiz-Ra wears a magical helmet that lets him control minds. When the time arrives and Wiz-Ra comes through the portal, Mumm-Ra steals the helmet. Cheetara helps Wiz-Ra get the helmet back and free the ThunderCats from its spell.

A vagabond space traveler lands on Third Earth to challenge Mumm-Ra to a fight. The Demolisher is victorious against Mumm-Ra and sends him running back to his tomb. Mumm-Ra acknowledges the power of the Demolisher, and convinces him to seek and destroy Lion-O. The Demolisher, searching for another fight, gladly accepts Mumm-Ra's advice. He battles with Lion-O only to face defeat.

A giant Ice King is accidentally released from his glacial prison and attacks the Castle of the Snowmen.

The Feliner is attacked by the RatStar, manned by the Mutants. Jackalman, Monikan, and S-S-Slithe are ejected and saved by Mumm-Rana, a female mirror image of Mumm-Ra, who is a force for good.