There's only one solid way to find out and it's after the jump! (Heavy spoilers apply! I am going to ruin the ending!).
So it's about little plant-people.
That's the short version. Now I'll go into more depth...
The Thundercats are still homeless and wandering the forest like a pack of hobos without a bindle. Lion-O is being all Lion-EmO and just can't get over the death of his fuzzy ginger father. Except instead of being all badass and vengeful, he's now totally pussied out of the fight and the Thundercats are fleeing from the rapidly approaching lizards, led by sleazy general Slythe.
I think Lion-Emo has given up on sticking his sword in Mum-Ra and is now looking for that dumb book. I hope he doesn't find it too easily because then the rest of the series will be him just sitting there reading it.
Anyway, the Thundercats soon stumble upon a grove of displaced tiny, cutesy, singing plant-people who are performing a funeral.
A particularly spritely baby one - I can't remember his name so let's call him Elmo - befriends Lion-Emo and convinces the cats to help them find a magic windy cliff and return them to their homeland. Lion-O agrees because I guess it's better than looking for some old book, right? So they gather up the smurf-like plant-people and more walking ensues.
But here's the twist. As the day progresses Elmo grows from a child, to an angsty teenager, to a young man. It seems that the plant-people's life-span is a mere day and he'll probably die in an emotionally charged scene just before they accomplish their goal. What seems like only hours to the Thundercats is like the passing of years for the Petalars, the message being that life isn't about achieving goals, it's about being cool and hanging out with your buds.
Okay... so it's a solid enough fantasy premise and, don't get me wrong, it's nicely done! The real issue I have is that once again our main characters take the backseat to the creature/problem of the week. I want to focus on the issues that the cats themselves have to solve and to learn more about them, but instead we're taking this full episode diversions to explore random whimsy that we'll likely never hear of again. Why?
Because as nicely crafted as this episode is (and it does pack a mild emotional punch) (maybe more of an emotional prod) there's virtually nothing about it that makes it specifically a Thundercats episode. You substitute almost any character for Lion-O, be it He-man, Sheera or Garfield, and the episode would probably play out exactly the same. Where's Tigra? Oh he's twiddling his thumbs in the background thinking about "hasing cheeseburgers" because he has shit all to do. All the epic promise of the opening two-parter continues to evaporate as this turns into a standard problem-of-the-week kid's cartoon show, and that just won't be enough to keep me going. If it's not intended for me then that's fine, but I'm a child of the eighties, and there was something about the beginning of all this that had me intrigued.
But wait! In literally the last 90 seconds the show apologises for its pussiness. Lion-O is fed up with all this shit and clearly worried that he's turning into a giant baby. So he goes screaming out of the woods, sword aloft and starts carving up the lizards like his next meal depends on it (and to be honest it probably does). He's surrounded by giant lizard powered mechs and it's looking a little grim for a moment and then - HOLY SHIT! WHAT IS THAT!?
THE GODDAMN THUNDER-TANK comes ripping out of nowhere, doing crazy donuts, and unleashing a torrent of lazers that blows everything up. Lion-O near craps his pants and then the top opens and a shadowy figure pops out and NUN-CHUKS a lizard in the gob like Michaelangelo squared! And Lion-O is all, "Who the hell are you? You're not an adorable button-cute plant person like my best bud Elmo" and then we see it's PANTHRO - who has been missing, presumed dead all series - and Panthro growls, "No, forget that pussy plant-baby shit. I'm the mighty Panthro! And I'm here to save you sissies from yourselves and ensure that people stick around and watch episode five!" And then the episode ends!
I'm rubbing my eyes! Did I dream that? I'd been lulled to sleep by the sweet lullaby of the hey-kids-let's-talk-about-your-mortality petalars and then Panthro burst into my room and nunchukkered my face in. So now I've GOTTA' watch episode five!
I'll tell you about it on the weekend.