The adventures of the pioneer Petya, the islander Doom and the scientific Soviet submarine in the southern seas. The boy Doom lived on the island, he was looking for pearls in the sea. Grandfather showed him the knife of their ancestor, Chief Batuallo. On the handle was scraped an island, and on it a mountain with two peaks. The grandfather said that there are many pearls around this island, and Doom sailed for a search. A thunderstorm began, and the boat dumped once to this island, but there a large shark attacked him.