Lion-O struggles with his destiny to become the next King of Thundera, as chosen by the Sword of Omens. Meanwhile, festivities are held to celebrate General Grune's return to the Cat Kingdom.

An unforeseen betrayal by a former comrade brings destruction upon the Cat Kingdom. While King Claudus leads the fight against General Slithe and his lizard army, the evil sorcerer Mumm-Ra attempts to seize the Sword of Omens.

After escaping their destroyed city with the Sword of Omens, the team encounters the Fishmen, who sail the quicksand sea in search of a massive sea creature. The ThunderCats' quest is to find the Book of Omens … but Lion-O is more interested in revenge on Mumm-Ra.

While Slithe and the lizard army hunt the Thundercats, they take refuge in a huge brier patch where the meet the Petalars, a race of flowering beings that despite living a short time, teach Lion-O a thing about life.

The ThunderCats accompany Panthro to Cloud Peak Mine to search for Thundrilium for the Thundertank. Along the way, Panthro and Grune's past is explored, from their first encounter to the reason behind Grune's betrayal.

The Cats look for the Book of Omens and navigate a dangerous labyrinth.

Lion-O is transported into the Book of Omens.

A duelist collects swords and sets his sights on the Sword of Omens.

The ThunderCats meet the mysterious beings who had helped them during the night and they turn out to be the Berbils, or Ro-Bears. The Berbils treat the ThunderCats with great hospitality until the village is attacked and some of the Berbils are captured.

Lion-O tries to reveal more about the Sword of Omens.

The gang come across a school in an enchanted forest.

Tygra's jealousy boils over, leading to a clash with Lion-O.

Lion-O and Tygra take on Mumm-Ra in a battle for the Spirit Stone.

Lion-O and the Thundercats try to convince the reptiles to join forces against Mumm-Ra and the ancient evil one is determined to stop them by adding two new generals.

The ThunderCats are ambushed by Slithe who is now assisted by Kaynar and Addicus. When Lion-O is knocked into the river and pinned down by the rocks, he finds himself in the spirit world where Jaga tells him that he must pass a bunch of tests in order to return to the living. The first test taking place in the simulation of Thundera's slums has Lion-O trying to figure out which Wilykit and Wilykat has the key to pass to the next test. The second test has Lion-O having to race Cheetara through a labyrinth.

Lion-O must finish his trials in order to rescue the other ThunderCats from Mumm-Ra and his army.

While looking for a shortcut through the mountains, Lion-O & Tygra find a long lost Tiger clan and even more surprising, Tygra's father is among them.
While seeking answers from his father, Tygra uncovers more than he bargained for.

With Tygra & Lion-O away, Cheetara & Panthro decide to teach Wilykit & Wilykat how to survive in the wild.
The lessons ultimately remind Wilykit & Wilykat about their times with their parents.

While looking for supplies in Dog City, the cats run across a flyer featuring fellow cat, Pumyra.
Lion-O decides to liberate her when it's learned she is a slave who fights for the amusement of spectators.

After Pumyra tells Lion-O cat slaves are toiling in the mines looking for the Sword of Plun-Darr, he, Cheetara, Tygra & Panthro split up to save the cats and uncover the sword before Ratar-O finds it and uses it to rule.

After learning the Sword of Plun-Darr has been found, Mumm-Ra goes after the Thundercats in order to gain it's evil powers.

Wilykat & Wilykit are still in Dog City to get their stuff back from the pickpocket Tookit who stole them. They join forces with other kids, who work for Tookit and live in "The Forever Bag", to ultimately stop Tookit.

The Thundercats require the assistance of a snake oil salesman named Ponzie to defeat Mumm-ra's latest monstrous form.

After making it back to the City of Dogs, Panthro is wondering why the Book of Omens is pointing to the sky. Lion-O runs into Jorma who had survived the attack on Thundera and has made it home. Upon studying the book, Jorma concludes that one of the stones the ThunderCats are after is somewhere in the sky. While exploring a secret scrap pile where new scrap metal falls from the sky, Lion-O, Tygra, Panthro, and Jorma are attacked by Necromech robots which end up stealing the Book of Omens. Upon tracking the Necromech robots, Lion-O, Tygra, and Panthro discover that they work for the Soul Sever (an alien turned robot by the Necromechs) who plans to use the power of the Book of Omens in order to find vessels for the three spirits of his deceased family.

The book of Omens leads the Thundercats to a city in the sky ruled by birds!

The ThunderCats must stop Mumm-Ra from getting the Tech Stone!