The Isle of Koumi, a beautiful island in the Pacific Ocean. On the island, people pass an old legend down from generation to generation that there was the Seabed Palace, an ancient ruin at the bottom of the sea, where the treasure of two female pirates, Anne Bonnie and Mary Reed (who really existed 300 years ago), was left. When Conan and his friends visit Koumi Island while on vacation, they meet some treasure hunters–and there was definately something suspicious..