The first rule of Thundrainium battle club is: you don’t talk about Thundrainium battle club.
We open on a dark and stormy night. Two bolkens are seeking shelter from the thunderstorm and see a giant tree in the middle of a field and decide that it looks like a perfect place to wait out the storm.
The settle down at the base of the tree, and moments later it is hit by lightning and splits in half.
They are unhurt, and discover that some steps had been buried under the tree.
One of them guesses that the steps lead to a tomb, which he considers quite lucky, since he’s decided to start an exciting new career in grave robbing. They enter the tomb, and discover a door with symbols on it, symbols intended to ward off evil (I suspect the big red X in the center is the most potent anti-evil symbol).
The bolkens rationalize that they aren’t evil, so there’s no reason they can’t bust in. They get a running start and actually smash through the door, which knocks the anti-evil symbol down and cracks it in half. They don’t find any treasure, but there are lit torches. With spooky blue flames! Still hoping for treasure, they push a giant stone slab to reveal an opening in the floor. Smoke rises up from the opening and solidifies into an anthropomorphic cat person with a single saber tooth.
Based on the saber tooth, the bolkens identify him as the legendary Groon the Destroyer, who ravaged Third Earth centuries ago. The bolkens figure out that they’ve unleashed his ghost. Groon is happy to be out, but feels weak. He wanders around and sees Cat’s Lair. He seems familiar with the design, but is surprised to see one here on Third Earth. Knowing that Cat’s Lairs are excellent sources of Thundrilium, he heads inside, phasing through the doors.
Inside Cat’s Lair, all the Thundercats are gathered in the control room, talking about the weather. A door opens behind Cheetara, and she notices a chill.
Tygra is quick to cast blame at Snarf for leaving the door open, but Snarf denies it, and points out that an open door wouldn’t account for the rapid drop in temperature. Which you would think Tygra would know, being the architect of Cat’s Lair and all. It isn’t like that door opens to the outside… Anyway, the door closes on its own, which Lion-O finds peculiar. There’s some loud banging, and some stuff hanging from the ceiling starts swinging ominously. Panthro is quick to dismiss it, though, and offers to turn up the heat. However, he’s thrown out of his chair!
Panthro, channeling Moe, wants to know who the wiseguy was. Cheetara informs him that it wasn’t any of them, there’s some alien force in there with them. Wilykat is quick to add that it’s stronger than Panthro. Panthro’s assessment of the odds that something is stronger than him? Not Likely. Tygra looks at the control console and calls Panthro’s attention to the fact that the gauges are going wild. He’s probably watched a lot of Gauges Gone Wild videos, so I trust him to know it when he sees it. A piece of machinery rips away from one of the walls, slides across the floor, and pins Panthro against a wall.
Tygra is concerned that the Thundrilium is being drained. Panthro orders Tygra to grab the controls, but when Tygra tries he can’t get to them. There are invisible hands in the way.
Lion-O and Cheetara help Panthro escape from the equipment that was pinning, but before he can do anything the console starts sparking, and the lights dim. Tygra figures out that whatever is doing this is drawing its strength from the Thundrilium. Groon semi-materializes to taunt them.
Cheetara and Wilykit try to attack him, but he uses teleportation and psychokinetic power to defeat their attacks. Wilykat tries to use his Lariat, but Groon transforms it into a burning ring of fire around him and Wilykit.
Lion-O tries to attack Groon with his sword, but Groon just teleports around to dodge the strikes. Groon finds this whole situation highly amusing. Then he grows to giganto-size.
Groon fades out, and then so does the scene. We cut to a wolo village, which Groon is giving the Godzilla treatment.
He decides that he’d like some Thundrainium, what Third Earthlings call “fire rocks.” Third Earthlings really suck at naming things. We cut to Cat’s Lair, where a coalition of Third Earth natives is petitioning the Thundercats for aid with the whole Groon the Destroyer situation.
The wolo spokesperson explains that Groon is demanding fire rocks, but they don’t have any. It’s been forbidden to mine fire rocks for a hundred mega-years (I wonder if a mega-year is longer or shorter than a galacto-year). Fire rocks were banned because their power was too difficult to control. Tygra notes the similarity to the Thundrainium they had on Thundera. Lion-O promises to help, although he’s not sure what they will do yet.
We cut to Groon, who has located some Thundrainium pits in a location that he’s already familiar with. Dude, if you knew where it was, why were you bugging everybody else about it? He forges a mace out of the stuff.
Back at the Cat’s Lair control room, the Thundercats discuss the situation. Cheetara points out the the ghost was familiar with Thundercats, but isn’t sure how. Lion-O figures they’ll need to know as much about him as he knows about them if they want to beat him. He tries to use the Sword of Omens’ power of sight-beyond-sight, but he doesn’t see anything. Tygra points out that this is because the Eye has no psychic powers. This gives Panthro an idea, so he throws a pointed look at Cheetara. She pleads with them to not make her do something. Apparently Cheetara has the power to receive psychic visions. Sometimes they just come, other times she has to bring herself to “the brink of oblivion” to get them. Tygra adds that it can take her weeks or months to recover. Lion-O gives her an out, but she volunteers to do it anyway. She stars spinning in a circle.
Her vision begins. She sees Groon, but he’s not the Destroyer, he’s… a Thundercat! (He also has two teeth in the vision). Furthermore, Groon used to hang with Jaga.
They were great friends, and great warriors, defending Thundera against all invaders. But Groon got greedy. Groon and his army of marauders swept across Thundera destroying and looting. Jaga pursued him, and they fought. For days. Jaga won. Groon was banished from Thundera and set adrift in space.
So, the Thundera branch of the ACLU kept this guy from getting the death penalty, and then the ancient residents of Third Earth had to deal with him? Nice going, jerks. Cheetara’s vision ends with the banishment, but they all figure they can fill in the rest of the details themselves.
Now, Cheetara’s vision was interesting and everything, but there wasn’t anything particularly psychic about it. Why couldn’t the sight-beyond-sight of the Sword of Omens have told them that? We’ve seen it show images from the past before. Does it have some kind of time limit on it, or something? And if this stuff on Thundera happened during Jaga’s lifetime, shouldn’t the other Thundercats like Tygra or Panthro at least know the history, even if it was slightly before their time? You’d think that an army of marauders sweeping across the planet would be the sort of thing that would make the news. Or was Jaga like a gazillion years old at the start of the series? Confusing.
Anyway, something starts shaking Cat’s Lair, and they all rush outside to see what it is. It turns out to be Groon the Destroyer whacking the heck out of the lair with his Thundrainium mace.
Lion-O tries to zap Groon with a beam from the Eye of Thundera, but Groon blocks it with his mace, which causes the beam to fizzle. They wonder how they can fight a ghost, but then Lion-O gets an idea. They can fight a ghost with another ghost: the mightiest Thundercat of all! “Thunder, thunder, thunder, Thundercats, HO!” Lion-O calls out for Jaga, who appears in the sky, similarly sized to Groon.
However, this has been what Groon’s been playing for all along. He wanted to call out Jaga so he could have a rematch. Groon points out his ace in the hole to Jaga: he forged his “battle club” out of Thundrainium. Tygra points out that Thundrainium weakens Thundercats, and Panthro adds that this will make it impossible for Jaga to win. OK, didn’t they establish that Groon was also a Thundercat? Why isn’t he affected by the Thundrainium? It can’t be because he’s a ghost, because Jaga is affected by it. Does Thundrainium weaken only Thundercats, but not non-Thundercat Thunderans? In any event, Groon and Jaga fight, and Groon manages to disarm Jaga.
Lion-O throws Jaga the Sword of Omens, which also transforms itself to giganto-size. They’re fighting, but Jaga keeps getting weaker due to the Thundrainium. Lion-O tells Jaga to take his strength. Jaga is reluctant, but Lion-O, as Lord of the Thundercats, commands it.
Jaga and Groon fight again, and Jaga is getting the upper hand this time thanks to the strength boost he got from Lion-O. Jaga knocks the Thundrainium mace into the sky. Groon surrenders, and promises never to fight Jaga again. Jaga puts the sword in his belt and turns to walk away, but Groon wants him to shake hands. But it was all a trick! When Jaga gets close Groon snatches the sword from Jaga’s belt!
“Your righteousness has made you strong, Jaga, but it has also made you foolish!” (I guess he subscribes to the Dark Helmet theory of goodness…) However, the Eye of Thundera makes a noise, and the Sword of Omens pulls Groon up into the sky and disintegrates him.
Lion-O congratulates Jaga, but Jaga attributes the victory to the strength of the Lord of the Thundercats. Jaga tosses the Sword of Omens back to Lion-O, and that restores his strength. Lion-O guesses that Jaga was onto Groon’s sword-snatching plan the whole time. Jaga tells him, “better an honest enemy than a false friend, Lion-O.”
In the coda, Tygra is working on a damage assessment of Cat’s Lair, and says there isn’t any structural damage, so repairs shouldn’t be too difficult. The bolkens from the top of the episode show up, and confess that they were the ones who set Groon’s ghost free. They say they didn’t mean to, they were just trying to rob his grave. They apologize for the whole mess.
Panthro is pretty angry, but Lion-O tells him to go easy on them since they made a mistake, and mistakes can be valuable if you learn from them. I don’t think I made any mistakes, but I know I learned something: only steal from graves that aren’t occupied by giant evil anthropomorphic cats.