Mumm-Ra learns of the legendary Treasure of Thundera and the Sword of Plundar, which destroyed Thundera. He and Ma-Mutt travel back in space and time to search the debris of the planet for the sword.

Exploring the newly reformed New Thundera, the Thundercats discover that Mumm-Ra is using all the Snarfs as slave labour to dig for the lost Treasure of Thundera.

Mumm-Ra retrieves the Sword of Plundarr as the Thundercats arrive on New Thundera. While the Thundercats try to retrieve the sword and treasure, Snarf tries to find a way to free his fellow Snarfs.

The Mutants arrive and Mumm-Ra has them lure the Thundercats to a strange place where, except for Lion-O, they are all transformed into child versions of themselves.

Lion-O frees the child Thundercats from Mumm-Ra's spell, and then confronts him to retrieve the fabled Book of Omens.

Mumm-Ra finds the Treasure of Thundera called the Totem of Dera, which has the power to heal and bring inanimate objects to life, and gives it to Vultureman that he might turn it into a device of destruction to use against the Thundercats.

Lion-O learns of the Chain of Loyalty, and Jaga instructs him to retrieve it from New Thundera. But when Mumm-Ra breaks the chain, the Thundercats turn on each other, and Lion-O must find a way to stop them from destroying themselves.

The Thundercats search a canyon of crystals for one of the Treasures of Thundera known as the Keystone, an item which has the power to heal, but also corrupts and enslaves the unwary.

Vultureman uses a new weapon on Cheetara to overwhelm her sixth sense and destroy her belief in herself and block her awareness of greater dangers.

The Thundercats capture the Lunataks and exile them to Exile Isle, an asteroid in deep space far from Third Earth. There the villains soon meet up with the Captain Cracker and hatch a plan of escape.

While attempting to decode the Book of Omens Lion-O inadvertently traps himself within it. Mumm-Ra later steals the book, later becoming imprisoned in it himself.

Tygra, Panthro, and Cheetara return to New Thundera to locate more of the Treasure of Thundera, but Mumm-Ra lures them to the mystical Canyons of Youth, which transforms them into children once again.

Alluro's new Superfuel formula for Sky Tomb's engines causes the local plant and animal life to grow to massive proportions. The Lunataks give it to the Mutants, who sprinkle it around Cat's Lair.

The Locket of Lies is a magical object created by Mumm-Ra to appear as a part of the Treasure of Thundera. When opened, the gems inside act as a compass that points the way to the remaining treasure.

When the Thundercats bring back more of the Treasure of Thundera. Snarf is fascinated by a bracelet and sees that when adorned, it, makes everyone obeys him. Mumm-Ra becomes aware of this, and will stop at nothing to make it his own.

The Lunataks hook up exercise machines to generate Sky Tomb's engines, but their own laziness forces them to kidnap the Thundercats and force them to do the hard work.

Lion-O recovers a mysterious telescope from the wreckage of the original spaceship. Mumm-Ra recognises it as the Thunderscope, which allows its user to locate pieces of the Treasure of Thundera.

Mumm-Ra discovers the power of one of the pieces of the Treasure of Thundera, the Jade Dragon, which possesses the power to entrap people in its stomach.

When an inter-stellar circus train comes to town, the Thundercats seize the opportunity to rid themselves of both the Mutants, and the Lunataks.

Fed up with Mumm-Ra's failures, the Ancient Spirits of Evil give him one final day to rid Third Earth of the Thundercats.