Carnage in C-Minor

The Decepticons are working on a plan that involves hurling a huge comet into Metroplex's face by installing giant thrusters into it, though the Autobots fly in after Perceptor had a hunch that insidious activity was going on in these areas, and the two sides start scrapping.
 

The Constructicons form Devastator, though this only leads into a confusing sequence where Broadside just falls on top of him from space while waving to Ultra Magnus, and then Brawn and a discoloured Huffer appear briefly before Ultra Magnus leaps upon Galvatron and the two grunt a lot behind smoke and fog, climaxing with Galvatron accidentally shooting the thrusters and blowing up the whole place.
 

However, this merely sends the comet hurling through space towards an inhabited planet, forcing the Autobots to try and stop it, though they're interrupted both by the Decepticons and also a strange sound eradiating from the planet that appears to stun them all, though it looks like they're getting funky. The sound also completely explodes the comet while drawing the Transformers towards the planet, landing in a forest-like area.

In the castle where the sound came from, it turns out that the sound came from three people: Allegra, Basso Profundo, and Zebop. Allegra gets inexplicably moody and runs away from the others, while the robots reawaken and continue scrapping, Devastator losing to Perceptor, of all people. It doesn't last long before Galvatron and Soundwave make an exit, hoping to find the harmonic sound that blasted them from space.

After the Constructicons shrink to fit in Magnus' hands, he, Blaster and Broadside set off to chase after the departing Decepticons, leaving Perceptor and Hot Spot to fend for themselves. Galvatron proposes that he and Soundwave stall the Autobots to get there before them, who have a jet to ride on, while Blaster notes that the civilisation of the planet all revolves around musical harmonies.

The Autobots find Basso Profundo and ask him to help in fighting the Decepticons, but he claims they shouldn't drag other people into their own affairs, and no one else will help them because he'll just kick them out if they disagree with him. Tough crowd. Naturally, Galvatron and Soundwave listen in on the conversation, and after hearing that Allegra is one of the three who hold the super harmony power, they set out to win her over to their side.

The Autobots head out to do the same, but don't get far before being crushed by a bridge.

Galvatron blasts the rubble just to ensure their death, but after the dead guys are badmouthed by Basso for causing destruction just by leaving the building, Zebop and his lackeys use the harmony to retrieve the Autobots from the debris. After being thanked by Magnus, Zebop claims he can help them find Allegra, who's elsewhere in her jamming studio doing a jig in front of a huge space age harp or something.

Galvatron and Soundwave interrupt her alone time by suggesting that if she comes with them, they can help her overthrow Basso Profundo's rule and basically help her get her own way, so she complies and allows Soundwave to record the harmony, though they're informed that this is only one third of it; Zebop and Basso have the rest.
 

Broadside ferries Ultra Magnus, Blaster and Zebop in search of Allegra's crib, only to find the two Decepticons testing out the third of the harmony on a cliff in the sea, finding even this fraction to be of admirable destruction power. The Autobots hide behind some bushes, waiting for the Decepticons to move away from Allegra before attacking, Zebop gives them away after hearing his love is allied with the bad guys.
 

Ultra Magnus lets Zebop enter his truck mode as he zooms forward to clobber the baddies, only to be pushed away by Soundwave's part of the harmony. Allegra tries to stop them from hurting Zebop, but seriously, they're multi-story tall robots, so that doesn't work too well, while her lover uses his part of the harmony to counter attack. This, however, only allows Soundwave to record it...

And use it and Allegra's third against him, causing the ground to break away beneath the Autobots and finish them off. Allegra doesn't take this lightly and uses the harmony to knock Galvatron down the pit as well, though he merely climbs out and finds her unconscious after being subjected to Soundwave's two thirds of the harmony, leaving him disappointed that she won't even feel any pain from dying.

The Autobots bust out from the rubble in the pit and start attacking again, allowing Blaster to get duelling with Soundwave and Zebop to rescue Allegra, though in the scuffle Soundwave is knocked down on top of Zebop, apparently killing him. Galvatron uses this opportunity to collect his blundering tape-filled idiot and vamoose.

Allegra grieves over the death of her love for a short while before using her third of the harmony to revive her harmony sharer, astounding the Autobots at the pure power of that thing. Back with Perceptor and the others, even the Aerialbots forming Superion helps little in the fight against the Constructicons and the not-supposed-to-be-here Soundwave, so they need help... and fast!

Basso Profundo watches the carnage from his city and uses the harmonic amplifier to combat it, though the most it does is disassemble Defensor, since it requires Zebop and Allegra to reach full potential. Galvatron and Soundwave arrive at the city...

And wreak havoc with the recorded two thirds of the harmony, destroying Basso Profunda's castle and prompting him to fight back with his third, which only ends direly for the citizens. The Decepticons flee to Earth to demolish Metroplex with their new weapon, while the Autobots arrive and are given the blame for the mess by Basso, though he does agree to help Allegra and Zebop in reviving their fallen friends.

The Decepticons arrive and set about attacking Metroplex with the harmony, rendering the big lug immobile but doesn't stop Scamper from talking too much and not looking like he's in severe audio weapon related pain. The Autobots arrive and deploy their secret weapon: the three harmony bearers singing their song with anti-sound, which prevents Soundwave from playing back his pain supplier.

Superion uses this opportunity to tackle Galvatron and allow Blaster to erase Soundwave's full harmony, forcing the Decepticons to make a hasty retreat. With the day saved, Ultra Magnus lectures Basso Profundo, Allegra and Zebop on how they should learn to live together in harmony, not squabbling and throwing people out of their homes for disagreeing with opinions, while Blaster quips "and that's the name of that tune!" Because he can.

Although just about every error in this episode is either mentioned or shown in the imagery (check the alt tags for further details!), there's still a bit of a scale problem.

Superion is a combiner, and therefore should be huge, but he's no bigger than Galvatron when he tackles him.

And Broadside, who looks entirely different in this episode, is meant to transform into a frigging aircraft carrier yet is only a few feet taller than Ultra Magnus. In his appearances with the entirely different robot mode, Broadside is as tall as the combiners and speaks with an accent I can't exactly describe due to my my lack of living-in-America-ness, while in this episode his voice is pretty plain. No idea if they're the same voice actor or not.

Ultra Magnus, Broadside and Galvatron are the only post-movie characters in this episode, with the likes of Rodimus Prime, Springer, Cyclonus and Scourge being strangely absent and completely unmentioned. This and the story's general silliness leads me to believe it was originally written for season 2, but was changed for season 3 by swapping out a few older characters for new ones; though how it would take place in space is something I've yet to fudge an explanation for, unless that's why it was bumped ahead for season 3. Or I could just be talking BS.