Ep 530. The King of the Fish-Man Island! Neptune, the God of the Sea!

In One Piece season 9 episode 530, titled "The King of the Fish-Man Island! Neptune, the God of the Sea!", the Straw Hat Pirates finally arrive at Fish-Man Island, a world full of sea creatures and mermaids. Upon arrival, they are greeted by the Fish-Man Island's leader, King Neptune, who is worshiped as a god by the other residents.

The Straw Hats quickly learn that the island is in danger, as a group of villainous humans has been causing trouble and stealing from the citizens. King Neptune's army of soldiers, the Sea Army, tries to keep the peace, but they are outnumbered and outmatched by the humans. To make matters worse, someone has stolen the island's most precious treasure, the Tamatebako, which was meant to be given as a gift to the World Nobles.

The Straw Hats offer to help King Neptune and his people, and they begin to investigate the situation. They soon discover that the culprit is a powerful fish-man named Vander Decken, who has a grudge against the royal family and wants to overthrow them. He has used his powers to throw the Tamatebako into the sea, which causes a chain reaction of events that threaten to destroy Fish-Man Island.

The Straw Hats must work together with King Neptune's family, including his sons, Princes Fukaboshi, Ryuboshi, and Manboshi, as well as the mermaid princess Shirahoshi, to stop Vander Decken and save the island from destruction. Along the way, they face many obstacles, including powerful fish-men fighters, deadly sea monsters, and treacherous underwater caves.

As they fight their way through these challenges, the Straw Hats learn more about Fish-Man Island and its inhabitants. They witness the discrimination and prejudice that the fish-men and mermaids face from humans, and they begin to understand why King Neptune has been leading a movement to promote peace and understanding between the different races. The Straw Hats realize that their mission goes beyond just stopping Vander Decken and saving the island; they must also help to bridge the gaps between the different worlds and cultures that they encounter on their journey.

In the end, the Straw Hats face a final showdown with Vander Decken, using all of their strength and teamwork to defeat him and save the island. They are hailed as heroes by the citizens of Fish-Man Island and are awarded the island's highest honor, the Medal of Valor. As they prepare to set sail once again, the Straw Hats reflect on their experiences and the lessons they have learned, ready to face whatever challenges and adventures await them on their journey to find the ultimate treasure, One Piece.