Conan, Kogoro and others led a large group to the beautiful island "Kamishima" in the Pacific Ocean for vacation. This island has been circulated since ancient times, probably about the ancient ruins "Underwater Palace" that sank to the bottom of the sea, and the treasure legend left by two female pirates 300 years ago. Xiaolan and Sonoko were not afraid of the rumors. When diving near the "Underwater Palace", they accidentally encountered a large group of hunters looking for treasures. Unexpectedly, one of them was suddenly attacked by a shark and died! Xiaolan and Sonoko escaped fortunately. Conan saw that the hunter died strangely and started an investigation with Huiyuan. Conan believed that the hunter's death had a lot to do with the treasure legend. He tried to solve the reason behind it. The mystery of the "Underwater Palace" has always been lingering in his mind. After he found a set of mysterious terms "JollyRoger (pirate flag)", he thought he had made some progress, but the hunter suddenly appeared and stole the treasure he had found! Xiaolan and Sonoko fell into the hunter's clutches, and their lives were in danger! What is the message hidden in the legend? What is the secret of the "Underwater Palace"? Conan vows to gradually solve the mystery of the code that travels through 300 years of time