Mumm-Ra hunts down the ThunderCats after taking on the form of an undead dragon-like demon named Sykalak, which allows him to fight even in the daylight.However, a supposed "Cure-All Elixir" Lion-O bought from a questionable Wolo salesman turns out to be an unexpected weapon against him.
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.
Mumm-ra and his forces are trying to get the Sword of Plunn-Dar from the ThunderCats. Lion-O and Pumyra act as the distraction for the lizards to follow in order for the rest of the ThunderCats to get back to the Thunder Tank. During the distraction, the origins of the Sword of Omens and the Sword of Plunn-Dar are revealed.moreless
Lion-O attempts to rescue a fellow ThunderCat from a fighting arena known as The Pit.
After falling to his death, Lion-O must pass a series of trials to return to life.
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.
While Lion-O and Tygra are in the Astral Plane dealing with their issues which have been magnified by the powers of Mumm-Ra who wants them to destroy each other, the other Thundercats protect the village from Slithe, Grune and the Lizard army. The Elephants make a surprising decision after meditating and Lion-O realizes the truth of being betrayed by Tygra. Panthro has some interesting flashbacks while telling Wily Kit and Wily Kat how Grune lost his tooth, and in the end Panthro makes a sacrifice to defeat his enemy.moreless
Using the Book of Omens to guide them, the Thundercats find themselves outside a mysterious stone enclosure. Entering in, the group discovers bipedal Elephants, far larger in size and stature than even Panthro. Lion-O introduces himself, and after a comical incursion, the Thundercats find the Elephants to be limited in the area of short term memory. Aburn and Anet, the two most enlightened elephants, try to offer advice to Lion-O and his group of outcasts. Aburn finds a common ground with Wily-Kit and Wily-Kat, and through the lullaby their mother sung them when they were young, Wily-Kit discovers her flute does more than just play music. Anet, meanwhile, tries to assist Lion-O in developing sight beyond sight. After an attack of what the Elephants call wraiths, giant hornet-like creatures, the Thundercats decide to do things their way…by initiating all out combat against the oversized insects. The Elephants take a different approach and meditate. After Lion-O defeats the wraiths, Anet and Aburn seem to become distressed. The sound of the giant insects kept an even greater threat at bay…a giant rock monster appearing to possess some limited magical abilities. Lion-O learns to utilize the harmony of the bigger picture, and when Wily-Kit and the Elephants defeat the stone giant with their music, Lion-O is given another opportunity to use sight beyond sight. This time, he sees more than just the Spirit stone…he sees its location. However, when he arrives in the Elephant's hut, the stone is not there. Anet and Aburn then advice him to take the Thundercats into the Forest of Magi Oar where the Sword of Omens' power can be amplified, and sight beyond sight can fully reveal the mystery of the missing stone…moreless
As Tygra and Panthro work on the ThunderTank, Lion-O is searching for supplies and encounters a mysterious Drifter who helps teach a few important lessons. After showing off The Sword of Omens in town, Lion-O is challenged by a man known as The Duelist and must overcome himself in order to win.moreless
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.moreless
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.
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.moreless