A monster called Mongor is accidentally freed.
The Lunataks lure Hachiman into a trap in order to steal the Thundercutter. The Thundercats have to help Hachiman regain his sword as well as his honour but a new enemy stands in their way.
The Thunderkittens feel ignored and unwanted so they run away from Cat's Lair to seek adventure. Upon their travels, they make a new friend.
The Thundercats set off to tackle some of the most dangerous criminals in the galaxy whilst Lion-O and Snarf hold the fort back at Cat's Lair. Mumm-Ra sees this as the perfect opportunity to attack and masterminds a plan to get inside Cat's Lair to wreak havoc.
Vultureman works with Chilla and invents a new device to get back at the Thundercats.
Mumm-Ra travels to Thundera to find the Sword of Plundarr and Treasure of Thundera. The Thundercats learn of Mumm-Ra's whereabouts and decide to build a booster rocket in order to fly to Thundera. Meanwhile, the Lunataks attack the Tower of Omens.
The Thundercats set off on their mission to Thundera. However, Vultureman resurrects the Ratstar and uses it against the Thundercats.
Mumm-Ra summons the Sword of Plundarr and continues his search for the Treasure of Thundera. The Thundercats land on Thundera and decide to recover the Treasure of Thundera before freeing the Snarfs. Snarf and Snarfer sneak off to free the Snarfs themselves.
Slythe and Vultureman crash land the Ratstar on Thundera where Mumm-Ra gives them the Book of Omens to decipher. The Thundercats pursue them and discover the Canyons of Youth where they are transformed into the Thundercubs. Meanwhile, back on Third Earth, the Lunataks attack Cat's Lair.
The Snowman helps to free the Thunderkittens who team up with the Thundercubs in an effort to rescue the enslaved Thundercats. On Thundera, Lion-O recovers the Book of Omens from Mumm-Ra's pyramid but Mumm-Ra isn't going to let Lion-O leave that easily.
Mumm-Ra and the Mutants use one of the Treasures of Thundera, the Totem of Dera, against the Thundercats. Lion-O becomes sick and only Snarfer can save him, with a little help from Jaga.
The Thundercats must return to the new Thundera to recover one of the most sacred pieces of the Treasure of Thundera, the Chain of Loyalty. Mumm-Ra disguises himself to help the Thundercats look for the Chain and break it which will make the Thundercats fight amongst themselves. It falls to an unlikely hero, Egbert, the Snarf, to save the Thundercats.