The Magic School Bus Gets Swamped is the fifth episode of season four and the forty-third episode overall. According to production order, this is the fourth episode of Season 4. It aired on October 11, 1997.
Carlos prepares for a debate over the proposal to build a new megamall on the wetlands. He's for building the mall, as he (and the rest of the class) see the wetlands as smelly and useless. When Ms. Frizzle's class arrives at the town hall so Carlos can debate another student, they find the opposition is none other than Janet. There's a coin toss to see who will argue for the mall, and she wins. He is shocked because he's never lost a coin toss. They break to prepare their arguments, him and the rest of the class returning to the wetlands. Carlos is even more shocked that he has yet to come up with any reason to keep the swamp where it is. Phoebe adds that a marshmallow plant (which Dorothy Ann reads in her guide to swamp that it grows in the wetlands and its roots made a sweet syrup, (which also makes the Mallow Blasters) could be one reason. With Carlos with one hour left to come up with a good argument why the swamp should stay where it's at, the Bus shrinks everyone but Arnold and Phoebe, the former happy about that, and they set out to find what's so special about the wetlands. The class then comes across a rodent, which is about five times their size.
The class runs away and realize that the animal is just building its own home. The class then comes across a bird that appears to be a marsh hawk, which swoops down to grab the class with its talons. The class narrowly escapes as the marsh hawk flies away. They also see that the marsh hawk is nourishing its offspring and Carlos becomes engrossed of all the amazing creatures that live in the swamp. A mosquito then flies in front of the class and a frog catches with its long tongue and proceeds to swallow it. They also come across ducks, which are called wood ducks. Dorothy also reads in her book that wood ducks eat mosquito larvae, nest their trees in the edge of the swamp, and use the swamp to raise their family. Carlos realizes it's not just a habitat but also a natural water filter. When Ralphie comes across oil in the swamp water, Ms. Frizzle transforms The Magic School Bus into a mega magnifier and the class climb aboard. Carlos feels that he's now ready for the debate, but Keesha tells him that the class needs more facts. Tim also notices that the swamp is cleaning the dirty water as it flows through and the swamp plants strain gunk out of the water. Carlos also learns that the swamp helps clean the water. Carlos notices bubbles floating in the bottom of the swamp and Liz activates the mega-mega magnifier to get a closer look to see bacteria that consists on water plants, fertilizer, and globs of oil. Keesha then learns that they are the Mike Robe gang, which turned her cucumber into a pickle (from the episode In a Pickle). Carlos learns that the gas comes out once the Mike Robe gang finishes eating the oil. The class also learns that the water goes through the wetlands and back into the river, as well as into the water works. Carlos is now prepared for the debate.
Ready for the debate, he and the class return to the town hall, but Janet's brash yet charismatic speech wins over the town. Phoebe stands up and argues that the wood duck can't live anywhere but the swamp, and Arnold joins her, adding that the marshmallow plant can't either. The others all stand, Wanda saying the smell is from the bacteria that cleans the water. They're reprimanded for being out of order, with Ms. Frizzle reminding them they have to obey the rules of the debate. Janet knows she's won. However, a lightning strike from the ongoing storm outside hits a dead tree, blocking the river from the swamp, and the town begins to flood.
Liz warns the others and the class wonder what caused it. So they hop onto the bus and find that the tree is what caused the flood. Using The Magic School Bus as a bulldozer, they quickly move to move the tree, and the flood drops. After learning the wetlands is what controls the flood, the town realizes just how much they need the swamp and Janet decides to have the mall built in another location. Ms. Frizzle concludes that "the wetland is the best land".
- The producer segment references both Airplane! and the spider episode, as a "viewer" tells the producer that "Surely it's not okay to pollute." The producer considers it a good point, and adds, "Don't call me Shirley," and the viewer tells him he already used that line in his spider show.
- The events from In a Pickle are mentioned.
- Ms. Frizzle quotes Lincoln Douglas for the second time in the series (the first time in Makes a Rainbow).
- Going by production order, this is the 4th episode of Season 4, and the 19th episode to be switched in streaming order form. This episode was completed on August 12, 1997.