In the series, Pirate Piet sets off on an exciting undersea adventure. At the beginning of each episode he leafs through his 'Great Wonderwater World Book'. In it he finds everything about the animals that live below and just above the water surface. He chooses a different animal each time, which he briefly discusses on the basis of a lifelike drawing. Then he dives into the depths of the sea in his submarine to look for it. With spectacular documentary footage from the BBC and some extra explanation from Piet Piraat, viewers learn everything about the animal.