The film follows the adventures of three children: a skateboarding mischief-maker named Fly, his sweet younger sister, Stella, and their cousin Chuck, a cautious, intelligent and overweight genetics prodigy. When their babysitter, Aunt Anna, falls asleep, the three children sneak off to go fishing only to stumble across the boathouse home of Professor MacKrill, an eccentric marine biologist. Reasoning that climate change could melt the polar icecaps within the next century, MacKrill has developed a potion that turns people into fish so they can survive the rising sea level and also an antidote to reverse the process. Unbeknownst to all, Stella drinks the potion, is transformed into a starfish and gets tossed out of the window into the sea. Since Stella's transformation was caught on camera, the tragedy is immediately discovered, so Fly, Chuck and Professor MacKrill head out onto the ocean in a desperate search. When a storm blows in, Fly recognizes the futility of their search, drinks the potion and jumps overboard, becoming a "Californian Flyfish". The boat capsizes and, because Chuck cannot swim, he's forced to drink the potion to survive, becoming a jellyfish. The Professor, the boat and all of its contents sink beneath the waves.