Quick recap
A year later, Billy and Sheena Deep are back on their uncle Dr. Deep's floating lab in the Caribbean. Billy can't stop pranking his sister, but this time the scares are real: a shark, a giant jellyfish, a huge snail, and his own goldfish, now the size of whales. The culprit is Dr. Ritter, whose plankton makes fish grow. He takes over the boat and forces Billy to drink the plankton, which turns people into fish. Billy had already swapped it for iced tea, so nothing happens. Ritter ends up swallowing the plankton himself, turns into a fish, and swims away. The twist: Sheena had swapped a bottle too, for a prank of her own. She drinks one and suddenly isn't sure she picked the right one.
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