The story of Excalibur, the greatest sword in history, is revealed to Mumm-Ra, who is then determined to find the Excalibur and defeat Lion-O and the Sword of Omens.

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

A Terator Starship, an extremely ugly spaceship, is pursued by a Kymera Craft, a visually appealing spaceship. The Terator Spaceship escapes and lands at the Cats' Lair

A spaceship freighted with a lethal radioactive sphere is on a collision course for Third Earth.

Vultureman invents a voice imitator that imitates the ThunderCats' voices. He uses it to lure the ThunderCats into perilous situations.

Mumm-Ra transforms into Mumm-Ra the Dream Master and summons spectral Dream Selves from the sleeping Panthro, Cheetara, Tygra, Wilykat, and Wilykit.

Willa is practicing her long-bow skills when a warning device alerts her to a Mutant attack. Willa is captured and the ThunderCats rush to rescue her.

Mumm-Ra summons the Mutants and threatens to banish them from Third Earth unless they bring him the Sword of Omens.

The concluding trial in Lion-O's Anointment Trials pits him alone and weaponless against the evil Mumm-Ra.

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.

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.

Vultureman invents the Shifter, a device that can switch personalities, and uses it on Panthro's personality and Snarf's body.

The Ro-Bear Berbils are a friendly group of robot bears that live on Third Earth. Lion-O and Snarf don't know this, and end up getting caught in a Ro-Bear trap. The Berbils free them however, and they form an alliance. Mumm-ra attacks the Berbil village, but the Thundercats save it. In return, the Ro-Bears help the Thundercats to build Cats' Lair, a new base for the Thundercats.

Bored and feeling that they are no longer needed, the Thunderkittens run away from home, but soon run into problems when they become ill from berries they had picked. A creature known as Cudi saves their lives. When the other Thundercats find them, the Thunderkittens invite Cudi to Cats Lair. But Mumm-Ra has been observing the situation and disguises himself as Cudi to gain entrance to the Lair. Many disasters occur and the blame is put on the false Cudi, who traps the Thundercats in the council chamber. The Thundercats realize it's Mumm-Ra and they manage to escape and defeat him.

On Thundera, Mumm-Ra creates the Locket of Lies and has Mu-Mutt plant it in the Valley of the Snarfs. Eggbert and Oswald Snarf find the locket and contact the Thundercats about it. When communications break up, the Thundercats (Lion-O, Cheetara, Panthro, Tygra) go to Thundera to see the Locket. When they arrive, the Locket leads them to a place in front of an old building. But the ground they are standing on gives way and they are trapped in a well. Mumm-Ra shows up and tells them not to try to escape, but the Thundercats try anyway. The walls start closing on them, and all their efforts to stop the walls fail. Finally, Lion-O uses the Eye of Thundera and the Cat Signal to push through the wall, and they all escape

When Snarf and a young unicorn are captured by the pirate Hammerhand and his crew of Berserkers, Lion-O goes to rescue them.

Wilykit and Kat find an ancient harp that has a genie in it. His name is Charr-Nin. Mumm-Ra appears to the genie and tells him that if he destroys the Thundercats, he will be freed from his eternal prison in the harp. Foolishly, Char-Nin agrees, and begins trapping the Thundercats in his harp one by one. Mumm-Ra's plan works, and he betrays Charr-Nin. During their arguing, the Thundercats manage to escape, leaving Mumm-Ra and Charr-Nin battling beneath the earth

The homeless Mutants also make plans to build their home. They enslave a primitive race of beings called Brute-Men to do their work for them. The Thundercats go to Castle Plunn-Darr to free the Brute-Men from the Mutants.

When a big storm hits Third Earth, the Thunderkittens are trapped in an old space capsule. Meanwhile, Tygra and Cheetra almost crash the Feliner, but Lion-O guides them in for a safe landing with the eye of Thundera. The kittens are captured by Mumm-ra, but end up saving the day by showing him his reflection in the window of the capsule.

Snarfer is making a trip back to Third Earth bringing food for the Thundercats, but he gets sideswiped by Chilla. Mandora, evil chaser sees this and pulls Chilla over. Chilla, however, ices Mandora and makes a run for it. Mandora insists on the Thundercats' help to capture Chilla. But when she and Lynx-O go to Darkside to capture Chilla, they fall into one of the Lunataks' traps. It seems that the Lunataks wanted the Braille board so that the Thundercats could not get to them. But when the board disappears, they try to get the information from Lynx-O as to where it went, but Mandora escapes and frees Lynx-O. She is tricked however and she and Lynx-O are shot up into space in a pod. Lion-O brings them down by using the Sword, and they return to Darkside and Mandora succeeds in capturing Chilla.

Vultureman creates a weapon that shoots pure Thundrainium in hopes of destroying the Thundercats. His first victims are the Thunderkittens who fall into a deep cavern and become injured. Lion-O and Snarf search for the missing kittens. Meanwhile Vultureman must get more Thundrainium from Chilla in Darkside and is also ordered by Slithe to fix the Mutants' machines. The Thunderkittens are saved by a Wollo and they discover that the Wollo village has been ""iced"" and they are captured and used as bait to lure Lion-O. When Lion-O and Snarf arrive, they are shot down by Vultureman's weapon and Lion-O becomes weak and summons the Thundercats. Just as Vultureman is about to finish off Lion-O, Ben-Gali arrives, wrecks Vultureman's weapon and drives the Mutants and Chilla away. Lion-O then uses the sword to restore the Wollo village.

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.

The normally submissive JackalMan decides to start his own army in hopes of conquering Third Earth by himself. He does manage to give the Thundercats trouble, but soon he is defeated and ends up crawling back to the other mutants at Castle Plundarr, who weren't very happy that he rebelled in the first place.

Mumm-Ra summons the Berserkers and coats their armor with Thundrainium and sends them to attack the Thundercats at the Tower of Omens. Realizing that their armor is coated with Thundrainium, the Thundercats at the tower need help and Lion-O and Tygra rescue them. They create a windstorm to rub off the Thundrainium and the Berserkers retreat. Meanwhile the Lunataks invade Cats' Lair and the Thundercats must rush back to save the Thunderkittens and Snarf who are being held as the Lunataks' slaves.

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.