Return of the Driller

Wilykit and Wilykat are enjoying themselves, picking apples in the forest. Mumm-Ra, who is observing them from his cauldron, is extremely annoyed that the ThunderCats are thriving on what he claims to be his planet. He summons the Driller and pays him diamonds to bore an underground hole from Acid Lake to the Cats Lair. The Driller complies by burrowing the channel and at once the lake's powerful Ultrasolvic acid starts dissolving the Cats Lair's foundation.

Unaware of the grave danger that they are in, the ThunderCats are carrying on their daily activities as normal inside the lair. As Snarf prepares apple sauce using the apples that the ThunderKittens plucked, the adult ThunderCats are testing out a new "Super fuel" invented by Tygra for the ThunderTank. Suddenly Lion-O detects a foul odor in the lair. Tygra puts the emanation through the analyzer and identifies it as being Ultrasolvic acid. Lion-O uses the Sword of Omens "Sight beyond sight" and discovers that the acid is right under the lair. Tygra suggests that the only substance that can neutralize the acid is Sponge Fog, and so the ThunderCats rush to obtain it.

The ThunderTank soon reaches the valley where Sponge Fog grows. Panthro engages the tank's cutters and carves out a block of the Sponge Fog. As the ThunderCats start to tow the Sponge Fog, Rockmen start pelting the tank with giant boulders from the top of the cliffs. In order to avoid the shower of boulders, Panthro decides to take a detour through theForest of Giant Insects.

As soon as they enter the forest, the ThunderCats first encounter the Mechanosect and then a giant beetle but the Cats manage to vanquish both the bugs. The tank then abruptly stops due to a fault with its transmission. Lion-O exits the tank to inspect it but is immediately seized in the mandibles of a giant worm. As the worm begins to burrow into its hole, Lion-O manages to grab his sword and summon the other ThunderCats. Panthro, Tygra and Cheetara attack the worm and it lets go of Lion-O.

Seeing that the ThunderCats had successfully overcome all the obstacles that they had faced, Mumm-Ra decides to delay them himself. He transforms into his ever-living form and flies out of his Black Pyramid. By now, the ThunderCats have reached the Bridge of Light and as they start to cross it, Mumm-Ra flies in and destroys the bridge. The ThunderTank falls into the River of Despair below. After almost being sucked into the Whirlpool of Infinity, the ThunderCats manage to reach the Cats Lair just in time to plug the hole in the foundation with the Sponge Fog and save their home.