It Was a Dark and Stormy Night when two hard-on-their-luck Bolkins try to take shelter under a tree, which is destroyed by a lightning strike to reveal a hidden stone staircase underneath it. They assume the steps lead to a tomb, and, apparently, the stories of what used to be buried with the dead in this part of the world haven't been lost to history because they conclude it must contain treasure and enter to see what they can find. The steps lead to a chamber with a giant stone slab they push aside, but instead of finding treasure, they release the ghost of a giant feline warrior with one sabre tooth whom they recognize as Grune the Destroyer, "the one who ravaged Third Earth" centuries ago.
All the Bolkins can do is run while Grune walks free from his prison. He's surprised to find a Cats Lair on Third Earth and, invisible, goes inside and starts attacking the Thundercats and wrecking their equipment so he can access the Thundrillium. The Thundrillium makes him strong enough to take on physical form and grow bigger. Like many ghosts, his abilities are the best of both worlds — livings beings can't hit him, but he can hit them. None of the Thundercats can stop him.
The empowered giant goes on a rampage, terrifying the residents of Third Earth. They beg the Thundercats for help, but they have no idea who they're dealing with or what to do. Cheetara decides she has no choice but to use her Sixth Sense — telepathic powers only Panthro and Tygra knew she had before now — to see the past, despite how the visions drain her. She sees that Grune was not only a Thundercat but Jaga's best friend and battle partner. Together, they successfully defended Thundera from all enemies... until Grune got greedy and decided he wanted more power. He led a band of marauders against Thundera, but Jaga defeated him and banished him into space. Somehow, he found his way to Third Earth.
While the Thundercats are learning about their new foe, Grune gets busy searching for a power source known on Third Earth as "fire rocks" — an unstable, dangerous substance that was forbidden long ago because its power can't be controlled. It's the same substance known on Thundera as Thundrainium — Thundercat kryptonite. Grune finds enough to fashion a giant spiked mace out of it and returns to Cats Lair with his new weapon.
Lion-O realizes that none of them can fight Grune, but he knows who can — Jaga. He uses the Thundercat roar to summon Jaga's ghost and gives him the Sword of Omens and his own strength to fight Grune. After a fierce battle between the two giants, Jaga manages to win, but Grune finishes himself off when he takes the Sword of Omens, which seemingly destroys him.
As the Thundercats set about repairing the damage Grune caused to their home, the two Bolkins who started the whole mess come to confess that they were the ones who accidentally released him. Panthro isn't inclined to forgive them easily, but Lion-O understands it was just a mistake and is impressed they owned up to it, causing Tygra to tell him he sounds just like Jaga.
This episode provides examples of:
- Batman Gambit: Lion-O quickly realizes that Grune wanted and expected them to summon Jaga to fight him all along.
- Blessed with Suck: Lion-O calls Cheetara's Sixth Sense a "gift." Cheetara calls it a "curse."
- Character Development: Up until now, the only times Lion-O has been seen taking charge are when he's alone or with the Thunderkittens. This is the first time he ever takes the lead among his adult peers.
- Cryptic Background Reference: It's not revealed just how much of the post-Apocalyptic state of Third Earth Grune is responsible for, but it's safe to assume he played a major part in it.
- Deader than Dead: Both Jaga and Grune are dead, but Jaga can still be hurt, and the sword does something to make Grune vanish when he foolishly tries to use it to attack a Thundercat.
- Defeat Means Friendship: After Jaga defeats him, Grune claims he's learned his lesson and now repents his evil... PSYCHE! Too bad for him Jaga knew he was faking when he tried to invoke this trope.
- Don't Touch It, You Idiot!: The door to the chamber where Grune was sealed is covered with ominous symbols the Bolkins assumed were there to ward away evil. Actually, the seal was there to keep something evil in and warn people on the outside "DO NOT ENTER! SOMETHING EVIL IS HERE! STAY AWAY!"
- Due to the Dead: One of the Bolkins objects to looting the tomb, despite his friend pointing out that no one would know.
- Give Me a Sword: When Grune disarms Jaga of his regular sword, Lion-O throws the Sword of Omens to Jaga.
- Hero of Another Story: We have no idea who defeated Grune in the past on Third Earth or how, but it must have been an epic story!
- Hoist by His Own Petard: Grune destroys himself by stealing the Sword of Omens, just as Jaga predicted.
- Honor Before Reason: Realizing Jaga won't be able to beat Grune even with the Sword, Lion-O offers to lend Jaga his strength, but Jaga refuses even as he begins to struggle.Lion-O: I command you, Jaga! As Lord of the Thundercats, I command you to TAKE! MY! STRENGTH!
- Ghostly Chill: The Thundercats feel the room grow cold when Grune first enters Cats Lair, before they even know he exists. They know the wind couldn't have caused it.
- It's Probably Nothing: When you hear an Evil Laugh while underground after opening a tomb, no, it's not "the wind."
- Magic A Is Magic A: Lion-O tries using the sword like a Crystal Ball, asking it to show him what happened with Grune in the distant past. It shows him nothing because Sight Beyond Sight doesn't work that way — it shows you the present or immediate events at the most, not the far past or future.Lion-O: The Sword shows me nothing.
Tygra: That's because the Eye doesn't have psychic powers.
[Everyone turns to Cheetara] - Mass "Oh, Crap!": The Wolos upon seeing a giant Grune coming to smash their village.
- Plot Hole: Zero explanation is given why Grune isn't affected by the Thundrainium even though Jaga is.
- Though he may had developed an immunity to it after betraying the Thundercats.
- Readings Are Off the Scale: Grune's presence has this effect in Cats Lair.Tygra: Panthro, the gauges are going wild!
- Sealed Evil in a Can: It held up for centuries until the Bolkins opened it.
- Security Cling: The Thunderkittens cling to each other when Grune first appears.
- Spikes of Villainy: Grune's uniform has spikes everywhere.
- Summon Bigger Fish:Lion-O: We fight a ghost with another ghost — the mightiest Thundercat of all!
- Takes One to Kill One: Grune is a ghost so the mortals can't hurt him, so Jaga, a ghost as well, is summoned to fight him.
- We Used to Be Friends: Jaga and Grune.
- Wham Line/Wham Shot: Cheetara sees Grune with the Thundercats symbol on his chest.Cheetara: I see... Grune. But not "The Destroyer". He's... a Thundercat.
Lion-O: Grune, a Thundercat?! Never! - The Worf Effect: The Thundercats can't tell at first what's invaded their home, but it freaks Wilykat out because "it's stronger than... Panthro!"
- Writers Cannot Do Math: Grune allegedly terrorized Third Earth "centuries ago." This is after taking the trip from Thundera to Earth, which is too long for a Thunderian to take when not a Human Popsicle. Yet, Jaga was still alive at the same time as Grune...
- Perhaps Jaga had a longer lifespan.
