Conan and his friends are on vacation on Koumi Island when they hear about the legend of the Seabed Palace. They meet a group of treasure hunters who are also interested in finding the palace and its treasures. However, Conan and his friends become suspicious of the treasure hunters because of their strange behavior and mysterious past.
As they investigate, they discover that the treasure hunters are actually former members of a notorious pirate crew. They were betrayed by their captain, who took all of their treasure and left them stranded on an island. With the help of Conan and his friends, the treasure hunters seek revenge against their former captain and hope to reclaim what is rightfully theirs.
Together, they embark on a dangerous journey to the Seabed Palace and encounter various obstacles and challenges along the way. The journey takes them through underwater caves and treacherous waters, but they ultimately reach their destination and find the treasure.
However, their victory is short-lived as they are confronted by their former captain and a fierce battle ensues. In the end, Conan and his friends help the treasure hunters defeat their enemy and secure their treasure, which they decide to share with the people of Koumi Island in honor of Anne Bonnie and Mary Reed's legacy.