Koumi Island, a beautiful island in the Pacific Ocean, is steeped in an old legend passed down through generations. The legend speaks of the Seabed Palace, an ancient ruin beneath the sea that allegedly houses the treasure of two female pirates, Anne Bonnie and Mary Reed, who lived 300 years ago. While on vacation, Conan and his friends arrive on Koumi Island and encounter a group of treasure hunters, whose motives appear suspicious.