Billy and Sheena Deep are back with their uncle for the summer, but Billy somehow feels that something bad is going to happen. The first thing that happens is that he imagines his sister is an octopus attacking him. Then he decides to scare his sister and make her think he's a shark. She freaks out, but she freaks out because there's an actual shark behind him. This shark "20 times as big as a normal shark." It kind of floats around and wanders off.
They go to tell their uncle, Dr. D, what they saw, but he doesn't believe them. He does give Billy some plankton to feed his fish and mumbles about some random fish he caught. Billy thinks it looks familiar and goes through a book. D tells him it's too big to be in that book but then realizes that it's a minnow. Even though minnows only grow like one inch long and this one is four foot long, it's a perfect match.
Billy wanders off to feed his fish and screams when he sees a head in the fishbowl. Sheena comes in laughing because it's just a doll head. D gets a kick out of it too, and laughs as Billy's fish go crazy over the plankton. Later on, the kids go swimming. Billy sees a weird pink blog that pretty much attacks her sister. When he tries to save her, he realizes that it's a massively huge jellyfish. He ends up swimming into the jellyfish, grabbing her, and pulling her out. Another jellyfish shows up and the two start fighting.
The kids run to find D and discover that he's missing. They then find a giant snail with D stuck underneath it. They get him out, and Billy realizes that the snail looks just like the one in his fishbowl but much larger. He hears a loud noise coming from his room and finds the goldfish have multiplied in size too. They end up throwing the fish over the side of the boat, despite them being freshwater fish. Suddenly, three random strangers show up on the boat.
The oldest man introduces himself as Dr. Ritter, a marine biologist working in the area. After some chit chat, he asks if they've seen anything strange. D tells Billy to shut up when he starts to explain. The giant snail then rolls out on deck and Ritter tells them that they know too much about his secret. He's apparently been doing weird things with the plankton. When the fish eat it, they grow in size. He plans on using it to end hunger.
Since they know too much, he has to get rid of them. He and his minions take them onto a different boat and tie them up. Ritter then tells one of the guys to throw Billy overboard. Just as he starts too, these massive birds appear in the sky. D points out that this what happens when other animals feed on the plankton and other fish. While the birds distract everyone else, Billy gets loose.
They start to run for the dingy, but one of the guys grabs a gun and fires. D slumps over in shock, but no worries, he didn't really get hit. They jump in the dingy and somehow manage to take off. They ride the boat to an island and try to make the best of it. They find a place to rest and some coconuts, but Billy wonders if they'll ever find a way home.
Sheena swims out in the water and catches a fish. Billy, being the typical big brother, decides it isn't big enough and tries to catch one himself. A giant crab, which probably came from my nightmares, grabs him and tries to pull him under. He gets free, and all three run and hide in a tree. A bunch of giant dolphins pop up and the crab runs away. The dingy rope gets wrapped around one of the dolphins, and it starts to carry it off. They all climb on the boat and let the dolphin drag them.
Billy flips out because it looks like the dolphin took them right back to Ritter. When he looks closer, he realizes that it took them back to their boat. As soon as they get on board, Ritter shows up again. He tells them that if a human eats the plankton, they'll turn into a fish. He makes Billy pick a bottle from the cabinet and drink it, but nothing happens. D attacks him, they roll around on the floor for awhile, and D comes out on top. Rather than go to the police, Ritter downs a bottle, turns into a fish, and swims away.
Billy confesses that he wanted to play a prank on his sister. He filled one of the bottles with ice tea and planned to drink it front of her to freak her out. When Ritter made him drink it, he picked the bottle he planted. Sheena giggles about how she did the same thing, grabs a bottle, and downs it. They suddenly hear her body making a weird noise, and Sheena wonders if maybe she picked the wrong bottle.