The Fishman Island Saga is one of the major story arcs in the One Piece manga and anime series created by Eiichiro Oda and the fifteenth season of the anime adaptation. This saga revolves around when the Straw Hat Pirates finally enter the New World into an underwater paradise known as Fish-Man Island, located 10,000 meters below the ocean surface.
The saga picks up when the Straw Hat crew reunites at Sabaody Archipelago after its two-year separation and training period. Their journey to Fish-Man Island introduces new challenges: first, they need to coat their ship so it could withstand such pressures of the deep sea. This process is a great representation of Oda's attention to detail in his world-building and specific challenges that come with navigation in the One Piece universe.
The Straw Hats get embroiled in local conflicts on Fish-Man Island. Most of them seem to surround the New Fish-Man Pirates, as led by Hody Jones, who advocate toppling down the island's royal family and taking revenge on human beings for discriminations meted against them centuries ago. Through all of this action, Oda grapples with issues concerning racism, historical injustice, and the cycle of hatred.
One of the key subplots is the prophecy by Madame Shyarly, a mermaid fortune teller, that Luffy will destroy Fish-Man Island. It sows tension, suspicion, and uneasiness regarding the presence of the Straw Hats on the island, thus putting complicating twists into their interactions with the local population.
The Fishman Island Arc opens up a stage for many core characters in the One Piece world. Notable additions include Jinbe, an ex-Warlord of the Sea who allies with the Straw Hats; Princess Shirahoshi, the giant mermaid with the ability to communicate with Sea Kings; and Vander Decken IX, a crazed fish-man in search of marrying the aforementioned princess.
Probably the greatest, most integral part of this saga would be the history of Fish-Man Island and the complicated relationship between humans and fish-men. We see, through many flashbacks, how Queen Otohime tried to bridge the gap between species and Fisher Tiger's violent rebellion against the oppression by humans. These backstories added depth to current conflicts and shed light on where the prejudice in the One Piece world really originated.
It would contain a number of famous fights that would prove how much the Straw Hats had improved after their two-year training. Luffy's fight against Hody Jones would show how he had finally mastered the use of Haki, and Zoro would have to fight a toxic octopus swordsman named Hyouzou, showing how strong he had become. Such battles would be action-packed, proving that they are ready for whatever New World has in store for them.
Another unique feature of Fish-Man Island Arc is the introduction of Energy Steroids, drugs that increase a user's physical power for a short time at the price of greatly accelerated aging. Here, it creates a time constraint for the conflicts to unfold while metaphorically symbolizing how hatred and its adherent revenge are self-destructive.
This saga also goes much deeper into the mysteries of the One Piece world—through the ancient weapon Poseidon, along with its relation to Princess Shirahoshi. That then ties back into the larger story about the Ancient Weapons and the Void Century, two of the series' major plot points.