With his fishing plans abruptly cancelled, impulsive teenager Fly finds himself stuck with Stella, his little sister, and precocious cousin Chuck at Aunt Anna's house. As one thing leads to another, the situation will take an unexpected turn when the kids chance upon the boathouse of the eccentric scientist, Professor MacKrill, and Stella accidentally drinks a yellowish potion. Now, as the children end up stranded in the deep blue sea, they only have 48 hours to find the antidote. Will the young adventurers remain fish forever?