Original air date: December 25th, 1992
Bright Eyes runs into the ice cream shop to inform her friends that there's a polluter going around polluting Ponyland, and that she saw a garbage truck at Big Pony River. They agree to split into teams of two to get to the bottom of the matter... but not before Bon Bon requests Starlight make her a triple dip double fudge banana split to go.
Clover and Bon Bon are hanging out at the river, and Bon Bon finishes her ice cream and throws the bowl and spoon into the river. Clover's chase after the bowl leads her and Bon Bon to an ice cream factory, and they sneak into the back of a truck to see where it's going. The containers inside don't contain garbage, but rather ice cream. While Bon Bon samples several of the flavors, Clover wonders what the driver could be doing instead of dumping waste. When the truck stops, the driver turns out be a mere delivery pony.
Meanwhile, Patch sees a barge leaving a factory and takes Sweetheart into a wooden boat to follow it. During the ride, Sweetheart removes a price tag dangling from her new hat, and Patch throws the price tag into the river. Believing the pony from the factory is bribing someone else to get rid of the garbage, she tackles the paying pony before hoofcuffing him. However, it turns out that this a recycling plant, so Patch's citizen's arrest is invalid.
Somewhere else, Starlight and Melody are on the prowl for the polluter before Melody decides to look at her new album, throwing the packaging into the river in the process. With their search unsuccessful, the two return to the ice cream shop. Bright Eyes regroups with her friends, but they all report they haven't found anything. Refusing to give up, Bright Eyes vows to continue the search tomorrow.
The next morning, Bright Eyes wakes up to find her room is full of water and her bed is floating on it. Bon Bon's father shows up, pulls her out of the window, and places her in a raft with her friends. He informs her that Big Pony River is blocked up and all of Ponyland is now flooded. When they reach the river, several ponies are working to clear the blockage of waste. Bon Bon's father dives underneath to help, and his daughter wonders who could be so thoughtless as to cause this mess. Just then, the girls all see the various things they tossed into the river the day before and realize they're the polluters. Just then, the river is unclogged and the water lowers. The girls then work to clear away the litter around Ponyland.
"Who's Responsible" provides examples of:
- Aesop Amnesia: During "A Lot Less Litter", Melody continues to litter even while her friends are working to clear it up. Subverted when a box she drops traps a bird, and at Bright Eyes' prompting, Melody picks it back up and frees the bird.
- Alliterative Title: Not the title of the episode, but of its song, "A Lot Less Litter".
- And I'm the Queen of Sheba: When the ice cream delivery pony catches Bon Bon and Clover in the back of the truck:Delivery pony: Just can't get enough of that pistachio, huh, Bon Bon?
Bon Bon: What? I'll have you know I'm on a mission!
Delivery pony: Yeah, right, and I'm the Christmas pony. - Bad Humor Truck: Subverted. Bon Bon and Clover get in the back of an ice cream truck, thinking it's full of garbage to be disposed of illicitly, but it's actually ice cream being delivered.
- Call-Back:
- Callousness Towards Emergency: Melody's response to Bright Eyes telling everyone about an incident of major pollution she witnessed:Oh, is that all? For a minute I thought it was something important.
- Chekhov's Gun:
- Bon Bon, Patch, and Melody all carelessly throw things into the river, and later find them again once the river is clogged, causing them to realize they were the polluters the whole time.
- At the beginning of the episode, Sweetheart shows off her new hat. Later, when she removes the label, Patch takes said label and throws it into the water.
- Earth Song: "A Lot Less Litter", during which the girls clean up the litter around Ponyland.
- Easily Forgiven: After catching Bon Bon and Clover in the back of his truck with the former sampling the ice cream, the driver cheerfully admits to having done the same thing himself from time to time and tells them not to do it again.
- Finishing Each Other's Sentences: The girls when they have their Heel Realization:Patch: I guess every little piece we tossed in...
Bon Bon: Became part of a bigger mess...
Melody: And finally clogged Pony River! - Foreshadowing: Bright Eyes is worried that the river could be plugged up by garbage, causing a flood. Guess what happens later in the episode.
- The Great Flood: The eventual result of Ponyland's wastefulness.
- Green Aesop: The girls look around to try and find the polluter responsible for the pollution across Ponyland, only to eventually realize it's their fault. They resolve to clear up the litter.
- Heel Realization: The girls realize they were the very polluters they were searching for.
- Heroic Wannabe: Not only do Bon Bon, Patch, and Melody worsen the pollution problem, but Bon Bon and Clover end up in the back of an ice cream truck they think is carrying waste and Patch attacks an innocent she mistakenly thinks is contributing to the pollution.
- Insane Troll Logic: Patch exhibits this:Patch: That's the local printing plant, right?
Sweetheart: Yeah, they make all the school's schoolbooks.
Patch: Exactly. And anyone who makes schoolbooks can't be completely on the up-and-up.
Sweetheart: What? But I like school.
Patch: See? They've even got you brainwashed. - Let's Split Up, Gang!: Clover and Bon Bon form one pair to go investigate the pollution, Patch and Sweetheart form another, and Starlight and Melody form the other.
- Off-into-the-Distance Ending: The episode, as well as the series itself, ends with the girls walking off into the distance down the street.
- Questioning Title?: Though there isn't a question mark at the end.
- Season Finale: As well as that of the whole show, since there were no more seasons.
- Series Continuity Error: Throughout the series, Patch seems to like to read and often talks about books. Then in this episode, she suddenly has a problem reading a sign that says "Recycling Plant".
- Spit Take: Patch spits out her drink when Bright Eyes warns her that pollution could make the water non-potable.
- Token Evil Teammate: Melody is a very downplayed example, wherein she dismisses the pollution matter as not important and later keeps littering even as her friends pitch in to clear up Ponyland.
- Trash the Set: In the finale of the show, Ponyland is flooded as a consequence of the pollution.
- What the Hell, Hero?: When Melody is dismissive of Bright Eyes' concerns about the pollution, Bright Eyes snatches away her magazine and scolds her.
