To prove themselves worthy of being ThunderCats, the ThunderKittens borrow the other ThunderCats' weapons and practice with them. They are ambushed by Mutants, who take their weapons.

Third Earth is experiencing a total lunar eclipse and Mumm-Ra uses this eclipse to cast a spell that will cause Cats Lair and everything in it to malfunction. The Thundercats first notice this when things start falling apart and fail. Lion-O and Panthro must fight the Mutants while having problems running the Thundertank. They still manage to defeat the Mutants, but crash the Thundertank. Lion-O realizes that this must be Mumm-Ra's evil work and goes to the pyramid to stop Mumm-Ra. Meanwhile Cats Lair is getting worse as Cheetara and Tygra rush to fix things in the lair but are attacked by the Lunataks. They defeat the Lunataks temporarily, but the Lunataks return to finish off the Lair. In order to break the spell, Lion-O must use the Sword to block out the sun

With only Lion-O and Snarf guarding the Lair, Mumm-Ra decides this is a perfect opportunity to invade the Lair, so Ma-Mutt captures Snarf and Mumm-Ra switches Snarf and Ma-Mutt. Ma-Mutt (who now looks like Snarf) goes to Cats Lair and gives Lion-O some poisoned candy fruit. Lion-O becomes ill and realizes that this Snarf is really Ma-Mutt and Mumm-Ra arrives and defeats Lion-O. Snarfer's bad dream of Snarf becoming a dog causes him, Ben-Gali, and Pumyra to go to Cats Lair, but Mumm-Ra lays a trap for them. He sends Ma-Mutt to get Snarfer and Lion-O becomes better again and distracts Mumm-Ra. When Lion-O summons the other Thundercats, Snarf (who looks like Ma-Mutt now), using Mumm-Ra's chant that changes Mumm-Ra into the everliving, becomes... Snarf-Ra the everliving and he rescues Ben-Gali, Pumyra and they in turn rescue Snarfer from Ma-Mutt. Then Snarf-Ra challenges Mumm-Ra and defeats him. After the battle is over, Lion-O uses the sword to restore Snarf to his original state.

Vultureman creates a device that plays around with Cheetara's sixth sense. She begins to see images that aren't real, and she soon begins to lose faith in herself. Meanwhile, the Lunataks enslave Panthro with a mind control device, and tell him to make the Sky-Tomb able to fly to Thundera. When the Thundercats are looking for Panthro, Vultureman makes Cheetara see images of him being captured by Mumm-Ra, being stuck on Hook Mt., and being held at Castle Plundarr. Lion-O goes to Mumm-Ra's pyramid, and is ambushed by Luna and Amok, who capture him. Tygra goes to Plundarr and is captured by Red Eye. The Thunderkittens also get captured on Hook Mountain. Cheetara is about to go crazy, but Lynx-O tells her to listen to her heart instead of her mind. She turns into a huge ball of energy that shoots out and destroys Vultureman's device and frees Panthro and the other Thundercats.

Aluro invents a ""booster"" potion in hopes that it can make Sky Tomb able to fly to Thundera. It doesn't work, and he throws the potion into the forest. Then he notices everything in the forest has grown huge. He and Luna give the potion to the Mutants, who sprinkle it around Cats Lair. The Mutants also capture Lynx-O and Snarf. But while the Mutants are preparing to transfer Lynx-O and Snarf from their cell, a mouse in the cell who Snarf had befriended gets some of the potion and attacks the Mutants. That lets Lynx-O and Snarf analyze the potion and come up with an antidote. Meanwhile, Lion-O, Pumyra, and the Thunderkittens are fighting off huge bugs in the forest that has grown around Cats Lair. Lynx-O and Snarf arrive, sprinkle the antidote over the forest, and then watch as everything turns back to normal.

Wily Kit and Kat are constructing a well for the New Cat's Lair on New Thundera. The Thundercats on New Thundera receive a distress signal from Thunderian refugees trying to find their way to New Thundera. The Kittens are left alone with Snarf on at the New Cat's Lair, and Lion-O and Snarfer are out constructing an early warning device (I guess for weather purposes). The rest of the Thundercats seek out the refugees in space. When Wily Kit and Kat strike water, they accidentally unleash a monster that Jaga trapped in the underground lake centuries ago. Frogman, with aquatic powers, tries to sink the New Cats Lair. Lion-O however re-traps Frogman in his watery prison.