While fleeing from a fierce thunderstorm, two Bolkins, Hurrick andBundun, seek shelter under a giant tree. Before they even get a chance to catch their breath, a bolt of lightning rips the tree apart, revealing a hidden chamber underneath it. Inside the crypt, the two Bolkins push a giant stone slab, thinking that there might be treasure buried underneath it but instead a ghostly apparition emerges from it. The Bolkins quickly recognize the ghost as being that of the legendary Grune the Destroyer, who ravaged Third Earth centries ago. Hurrick and Bundun flee for their life as Grune makes his way out of the tomb.
Looking for Thundrillium to replenish his strength, Grune stumbles upon Cats Lair. Inside the lair, the ThunderCats start witnessing paranormal activities such as drops in temperature and strange noises. A piece of the lairs' wall rips away and pins Panthro against a wall. The lair's Thundrillium supply then starts draining at an accelerated rate. Tygra tries to stop the outflow but is unable to. Suddenly, Grune's ghost materializes in front of the ThunderCats, taunting them. The Cats try to fight him but it is in vain as Lion-O describes it as "fighting air". Grune then grows to an enormous size and before disappearing, he promises the Cats that they will meet again very soon.
The next day, Grune terrorizes the various peaceful inhabitants of Third Earth such as Wolos, Bolkins and Berbils by flattening their villages. The Third Earthers come to the ThunderCats, asking to be rescued from the menace. They also inform the Cats that Grune was demanding "Fire Rocks". Hurrick and the Wolo Traveller point out that they didn't have any as it had been forbidden to mine them for over a hundred mega-years due to their power being too difficult to control. Tygra recalls that they had a similar element on Thundera known as Thundrainium. Lion-O promises to help the Third-Earthlings even though he is not quite sure how.
Meanwhile, Grune had managed to locate some Thundrainium and forges a Battle Club out of the substance. Back at the Cats Lair, the ThunderCats are assessing the situation, wondering how the ghost knew so much about them. Lion-O tries to use the Sword of Omens' power of Sight Beyond Sight in an attmept to find out more about Grune but sees nothing. Tygra points out that it is because the Eye of Thundera has no psychic powers. Eventually, it is up to Cheetara to use her Sixth Sense and in so doing, she reveals that Grune used to be a ThunderCat who fought alongside Jaga to protect Thundera against all invaders. But greed overcame Grune and he started destroying and looting. Jaga intervened and after a lengthy battle between the two, Grune was banished from Thundera and set adrift in space. Panthro guesses that Grune's spaceship must have somehow found its way to Third Earth and that is how he ended up there.
Suddenly the whole lair starts shaking and when the ThunderCats go out to investigate, they see that a giant Grune is pounding at the lair with his mace. Still not sure how to attack Grune, Lion-O stumbles upon the answer to the mystery."We fight a ghost, with another ghost. The mightiest ThunderCat of all!", he proclaims and uses the sword to call forth the spirit of Jaga. Grune is pleased as that was his plan all along, to draw out Jaga so that he could have a rematch.
The two spirits engage in battle but Grune's Thundrainium mace weakens Jaga and he manages to disarm the ThunderCat patriarch. Lion-O throws his sword to Jaga and the battle resumes but Jaga is still continuing to get weaker due to the effects of the Thundrainium. Seeing this, Lion-O tells Jaga to take his strength. Jaga is reluctant, but Lion-O, as Lord of the ThunderCats, commands it. Armed with Lion-O's strength, Jaga defeats his opponent with ease. Grune, then pretending to offer his hand in friendship, grabs the Sword of Omens from Jaga. However, the sword pulls Grune up into the sky and disintegrates him. Jaga returns the sword and Lion-O's strength back to the young lord.The next day, Hurrick and Budun come to the Cats Lair and reveal that they were the ones responsible for freeing Grune and that they were sorry. Panthro is angry at them but Lion-O tells him to take it easy and points out that even though the Bolkins made a mistake, mistakes can be valuable if one learns from them.