It was another peaceful day on board the Going Merry. Nami yelled at Luffy for whizzing in the ocean. As it'll just contaminate the fish that are swimming. Luffy was on fishing duty, but he was using sponges for bait, which he sees them to look like hamburgers. Nami couldn't believe how naive Luffy is, until he caught something big, that it dragged the ship backwards instead of forwards. The fishing line ended up at a rocky crater with a chest being chained to it. Luffy was disappointed that it wasn't something to eat. But Nami was more interested in finding out what's inside the chest. She opened it only to find just a skull inside. Usopp read the back of the skull, which engraves about a treasure that can make anyone's dream come true. It lies deep in the ocean's naval. Then a large hole began to form near the crater they crashed into. So Usopp, Nami, and Sanji prepared themselves in a diving barrel to explore the cavity in the sea. Luffy was put in charge of the rope crank. It was a smooth ride, for the trio in the barrel. Right until Luffy got side tracked and found a flying message in the bottle. Which he became disappointed that it wasn't a drink. Then the diving barrel began to fall faster into the naval. Usopp used the hose communicator to warn Luffy, that their in danger. Luffy managed to get hold of the rope. And the barrel barely crashed into the surface of the naval. Usopp, Nami, and Sanji find themselves in a ship graveyard. And were greeted by a laughing bat who says he'll guide them to the treasure. The treasure that Captain Joke tried to steal one decade ago. And they must defeat the 4 large aquatic monsters that were awaken when Joke tried to steal it for himself. Will Luffy and Zoro make it in time to aid their friends? Will they be able to defeat the mammoth monsters? And who is the bat who is driving everyone batty with his laugh?