"Goes to Mussel Beach" is the third episode of season four and the forty-third episode overall of the animated children's series The Magic School Bus. It originally aired on September 27th, 1997.
The class has gone to the beach, but Ralphie insists that they stay in one spot even though it is crowded and trashy. The rest of the class thinks it is a rotten spot on the beach, but he insists that it is where they want to be.
Ms. Frizzle arrives via surfboard and receives a letter that float in with the tide. Tim understands what Ms. Frizzle means and at a pier, the water rises up when high tide and down when low tide. The letter Ms. Frizzle received is from her Uncle Shelby. The letter explains how he needs a special spot saved for a friend. Eager to get away from Ralphie and explore the rest of the beach, the class, minus Liz and Ralphie, go to find the spot.
Wanda checks the map and realizes that the class' destination is underwater and Ms. Frizzle transforms the bus into a submarine, which she and the rest of the class take a ride in underwater. The teacher then transforms the rest of the class into mussels and herself into a crustacean. The class then finds that the inhabitant of the spot, Mert, is not a human, but a mussel.
Other ocean creatures, such as turban snails and goose-neck barnacles, have taken over Mert's spot, and a lottia gigantea (otherwise known as an owl limpet) pushes the rest of the class out the way. So the class decides to find him a new spot. The middle zone is where the mussel made his home originally.
They try to go to the low zone, but find worse accommodations there. Sea stars try to eat them! Mert would never be safe there. Ms. Frizzle then equips her little mussel students with jet packs so they will be able to move around on their own. They head to the high zone. However, at low tide there is not enough water for mussels to breathe and there are hungry seagulls above looking for snacks.
The class decides to go back to Mert's original spot, which is just right for him. The high zone is to hot and dangerous while the low zone was replete with starfish ready to eat the mussels. With the help of the bus, in the form of an owl limpet, they push everybody else out. Mert gets returned home, thanks to a phone call from Dr. Shelby, and the class goes back to Ralphie who has come up with innovative solutions to survive the beach spot he chose.
When they get back to Ralphie, they apply what they learn about finding the right place to live to try to convince him to move. They then find out his reason for picking the spot he chose: it makes them first in line for ice cream.
- Uncle Shelby's phone voice is actually Ms. Frizzle's dialogue sped-up going like this:
"Oh, I got his machine. Val here, uncle Shelb. Tell your friend, Mert, we'd be happy as clams to watch his place for him. Contact with you later."