Billy and his sister, Sheena, are visiting their uncle Dr. Deep on a tiny Caribbean island. It's the perfect place to go exploring underwater...and Billy's ready for an adventure. There's only one rule to remember: Stay away from the coral reefs. Still, the reefs are so beautiful. So peaceful. Billy can't resist. But he's not alone in the water. Something's lurking deep below the surface. Something dark and scaly. Something's half-human, half-fish...