The first season of ThunderCats began airing on July 29, 2011, on Cartoon Network in the United States. The series is a reboot of the original series of the same name created by Tobin "Ted" Wolf. The first season aired 26 episodes.
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 traveling through the woods, the ThunderCats come across a group of Petalars - small plant-like creatures whose lifespans are only one day. Lion-O befriends the young Emrick as he and the others try to figure out a way out of the woods and return the Petalars to their home. Meanwhile, Slithe and his lizard army are hot on the ThunderCats' trail to eliminate them.
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 ThunderCats continue their quest for the Book of Omens, which leads them to the fabled Tower of Omens. However, they will first have to get past a series of deadly traps in order to reach it. Meanwhile, Mumm-Ra grows tired of waiting and forces Jaga to reveal the Book's location, and plans to go there himself to take it at all costs.
Lion-O travels into the Book of Omens to live out his ancestor's past as Leo, who led the rebellion against Mumm-Ra millenia ago.
The origin of the stones and the cat's relationship to Mumm-Ra and the other animals is finally revealed!
While traveling through a village built on swordsmanship, Lion-O's cockiness loses him the Sword of Omens to the Duelist, a villainous sword-collector.
With the help of the listless Drifter, Lion-O must get it back.
Tiny robot bears solicit the help of the ThunderCats to save them from a slave trader who values their technical and construction skills. The Cats help to rescue enslaved Berbils and defend their village. Panthro learns that's he's not the only "wrench monkey" and bonds with Robear Bill.
The Cats track the location of the first stone to the mystical Elephant Village, but Lion-O can't find the specific location and the forgetful Elephants no longer remember where it is. Lion-O must learn to use Sight Beyond Sight and interpret the consequences of his actions when he inadvertently invites an attack on the village.
Sent by the Elephants to the Forest of Magi Oar, Lion-O must learn to master Sight Beyond Sight before returning to the Elephant Village. The Cats encounter a group of paper warriors with a dark secret.
The Cats are ready to retrieve the stone from the "Astral Plane" in the Elephant Village, but find it overtaken by Grunea and Mumm-Ra's forces. Plus, Tygra and Cheetara's early relationship to one another is revealed.
Tygra and Lion-O enter the Astral Plane to retrieve the stone, but encounter Mumm-Ra in the process. Grune and Panthro have their final duel. Cheetara finally chooses her beau.
The Thundercats go on the offensive taking down Mumm-Ra's lizard army piece by piece. Lion-O appeals to the soldiers to turn against their overlord, but Mumm-Ra won't see his army crushed so easily. He tasks Slithe with recruiting two new and vicious Generals -- Addicus and Kaynar -- to retaliate against the cats. Meanwhile, the Robear Berbils make Panthro a new set of robotic arms and soop up the Thundertank.
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. Back in the living, Tygra and the other ThunderCats managed to get free from their shackles, but Slithe, Kaynar, and Addicus escape with the Sword of Omens. Tygra vows to get the Sword of Omens back.
The other cats remain trapped in Mumm-Ra's Pyramid and, believing Lion-O is dead, attempt to escape to prevent Mumm-Ra from getting the Eye of Thundera out of the sword. Lion-O has two trials left: Panthro and Tygra, but he fails to defeat the latter. Pleading for a chance to save his friends, Jaga grants him life until sunrise to try and save his friends … and stop Mumm-Ra.
While traveling through snowy mountains in search of safe passage, Tygra and Lion-O come upon a lost tiger village and in it they find Tygra's real father. Tygra learns why his father sent him away as a baby — to save his life. But all is not as it seems in this village as the brothers learn these tigers are cursed and Tygra may be the answer to saving them — but at the cost of his own life.
While Lion-O and Tygra were off with the tiger clan adventure, the rest of the crew is keeping an eye on the Thundertank. The twins are looking for some stashed food, but it appears Snarf beat them to the goodies. Panthro and Cheetara decide to use this as an opportunity to teach these two how to survive on their own.
Lion-O and company come upon a Dog City where they unexpectedly find a cat: Pumyra. This comes as a surprise since they believed all the cats were enslaved by Mumm-Ra, but an even greater surprise is that this cat has no love for Lion-O and, in the end, he's forced to face the cat down in a death match — and she's not pulling any punches. Meanwhile, Panthro runs into an old friend turned enemy, Dobo.
Arriving at Mount Plun-Darr, the ThunderCats and Pumyra discover that the Thunderian slaves are being forced to excavate the Sword of Plun-Darr for Ratar-O and his Rats.
As Lion-O and Pumyra try to evade Mumm-Ra and his forces in the mines of Mount Plundarr in an effort to keep the Sword of Plundarr out his hands, we flash back in time and discover its origin … and its relationship with the Sword of Omens.
Kit and Kat follow Tookit into his magical Forever Bag - a seemingly normal bag that is infinitely big on the inside - where they meet a group of orphans. Seduced by the their fun loving lifestyle of Tookit and the orphans, Kit and Kat are tricked by Tookit into becoming thieves again. Now they must turn the tables on the charming rogue Tookit or end up in jail.
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!
In the season finale, Mumm-Ra’s forces find the ThunderCats and the Tech Stone in Avista City. An incredible battle ensues for the stone as Lion-O finds himself betrayed by a trusted ally.