"Take a Dive" is the thirteenth episode of season four and the series finale of the animated children's series The Magic School Bus. It originally aired on December 6th, 1997.
Upon discovering Ms. Frizzle's ancestor was Red Beard the Pirate, Wanda wants to set off to find his treasure. Dorothy Ann, who was her partner for a class project, is upset that she did their project on her own, and she's even more upset when Wanda doesn't want to work together to find the treasure, as the other pairs are all working well together (Keesha and Ralphie, Tim and Carlos, and Arnold and Phoebe). When Ms. Frizzle comes around, she shows the class the map of her great grandfather Red Beard, who was the pirate. Wanda is engrossed and is convinced that there is real buried treasure at the ocean.
Ms. Frizzle leads the class to the bus, which transforms into a ship, and they navigate through the ocean. When they get to their designation, Liz drops the anchor and the class partners up. After they dive in to the water, they notice that fish are absent in the deep end, and D.A. reads in her book that there's scanty food and shelter for fish to survive in the deep end. Wanda reads the map and the class comes across a school of fish and the coral reefs. Wanda and D.A. swim to the coral reefs and the two spot a gold chain with a pirate design. Wanda is convinced that the chain leads to the treasure and attempts to swim to it. But D.A. stops her, reminding her that the two are partners. When Wanda asks what good a partner would do, the bus changes everyone in to animals that have natural partnerships, much to Wanda's dismay.
Half of the class have been transformed into sea anemone and D.A. reads in her book that it is a sea animal that looks like a plant. The rest of the class is hermit crabs. Since Wanda is an anemone and can't move, D.A., as a hermit crabs, attempts to follow the chain, only to get suppressed by a crowd of fish. The class also find that the fish don't swim near anemones because they sting them. Tim notices a hermit crab placing an anemone on its back to protect itself from an octopus. So the students who are hermit crabs place their partners, who are anemones, on their backs. Wanda conceives the idea that D.A. could place her on her back so the latter could get to the treasure. Liz they arrives with the bus and transforms the class into pistol shrimps and Goby fish. They manage to dig up the hole and hide inside to protect themselves from an upcoming shark. They crawl through the small opening where the chain leads to and follow it. Since Wanda can't see, Liz uses the bus to transform the students into sharks and remoras, who attach themselves to sharks. The class finally finds the treasure, only to find that it has damaged the coral reef. D.A. and Wanda inadvertently cause an avalanche attempting to haul the treasure and the bits and pieces of the reef fall apart.
The two swim out of the fallen bits of reef and the treasure is buried under it. Liz uses the bus to transform everyone back to their human form and Ms. Frizzle offers the students their magnifiers. Carlos notices that the reef is made of tiny animals called polyps. Tim and Wanda both notice a substance the polyps have on each of them, which Ms. Frizzle informs is called algae, the same as the scum on Arnold's shoe when he stepped into the pond (from the episode Gets Eaten). After remembering the animals they were to make great partners, the class wonders if algae has any partnership with the polyps. Ms. Frizzle explains that algae makes its own food from sunlight and gives out wastes to help the polyps make the reef. Ralphie then notices that the sick part of the reef has no algae partners and D.A. reads in her book that the polyps get rid of their algae partners once the coral reef gets polluted. The class then digs up the treasure from the bits of coral and learns that the treasure has been polluting the reef due to its copper chest. Thanks to the bus, the class manages to restore the reef, giving the polyps to partner with the algae, and take the treasure home.
When they get back inside the classroom, they open the chest. Inside is a portrait of Red Beard and a model of his ship, a "Magic Spanish Galleon," as Ms. Frizzle calls it.