Help! I'm a Fish

On a trip to the seashore with his little sister Stella and their cousin Charles, the impetuous Fly discovers the secret laboratory of Professor Crevette in a cave. This brilliant scientist has invented a potion to transform humans into fish. Believing that it is just lemonade, Stella drinks some and finds herself at the bottom of the ocean, in the shape of a starfish. To find her, Fly and Charles take the potion too, and turn into a flying fish and a jellyfish. Their rescue mission becomes complicated when the antidote falls into the Machiavellian fins of two fish who, through drinking the potion, have acquired enough intelligence to want to dominate all their fellow marine dwellers.