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Two years after the Paramount War, each member of the Straw Hats returns to Sabaody Archipelago. However, a group calling itself the Straw Hats is having everything their way and is recruiting new members.
The Straw Hats finally set up a plan to leave Sabaody Archipelago, while the fake Luffy tries to punish those who insulted him. Things get even more chaotic when the Navy raids both Brook's concert and the gathering of the fake Straw Hats.
The Straw Hats finally reunite after two years. With the help of all their supporters, they set sail for Fish-Man Island. While admiring the underwater scenery, Franky begins to tell the story of a man who protected the Sunny all this time.
With Caribou and his crew hot on their tail, the Straw Hats descend to the depths of the sea to get to Fish-Man Island. And as they're about to ride the downwelling current that will take them further into the depths, a legendary giant creature rears its ugly head.
As the crew plumb the depths of the sea in search of Luffy, Sanji and Zoro, they encounter the terrifying and mysterious secrets of the deep, including an undersea volcano, whose eruption almost spells "the end" for the Straw Hats as they continue on their way to Fish-Man Island.
The Straw Hats finally land on Fish-Man Island, but they get separated. Luffy, Usopp, Sanji, and Chopper reunite with Camie and she leads them to the Mermaid Inlet where Sanji's "enthusiasm" gets them into trouble.
Madame Sharley, Fish-Man Island's popular prophet, has a premonition that may spell the end of the undersea kingdom; the true power and intentions of the New Fish-Man Pirates are made clear.
Luffy's group reaches Ryugu Palace, while Caribou kidnaps a group of mermaids after being accidentally freed. Learning of the kidnapping and Shyarly's prophecy, Neptune's forces attempt to arrest the Straw Hats while Luffy wanders off, finding and befriending the Mermaid Princess Shirahoshi. Elsewhere, Vander Decken IX meets Hody Jones, solidifying their alliance to take down the Ryugu Kingdom.
Straw Hat crew accidentally occupy Ryugu Castle and take King Neptune and the others hostage. And while Hody Jones and Vander Decken's ingenious plan to infiltrate the castle is put into action, Luffy takes the Mermaid Princess for a walk.
Hody Jones begins his attack on Ryugu Palace, where he is challenged by Roronoa Zoro. Meanwhile, Luffy and Shirahoshi continue on their journey, but Megalo is unable to keep hiding the Princess. She is discovered by the kingdom's residents.
Neptune's army with Keimi and Nami manage to leave the Ryugu Palace, which is besieged by Hody Jones; Zoro, Usopp, Brook, and King Neptune are forced to stay. In the Sea Forest, Luffy joins Jinbe, who reveals a shocking truth to him.
Jinbe tells the story of Queen Otohime and Fisher Tiger. Sixteen years ago Shirahoshi's mother, Queen Otohime, tried to convince her kind to live in peace with humans. Fisher Tiger, on the other hand, attacked Marie Joie to free the slaves.
The Sun Pirates return Koala to Foolshout Island, but the Marines were waiting for them, and Fisher Tiger dies in the fallout. In the Ryugu Kingdom, Otohime's efforts seem to result in failure, until a World Noble crashes on the island. Visiting Mary Geoise with him, Otohime convinces him to join their side, signifying the first big victory of her pro-peace movement.
Jinbe completes his story, culminating in Otohime's assassination. In the present, Hody broadcasts himself across the island, declaring a new kingdom based on fish-man superiority and threatening the Straw Hats. When Luffy plans to go rescue Hody's hostages, Jinbe tries to stop him, and the two pirates prepare to face off.
The New Fish-Man Pirates defeat swarms of the Royal Army and assemble at Gyoncorde Plaza, where they plan to execute King Neptune. There, they are attacked by the princes, and each of the crew's officers take Energy Steroids in anticipation of fighting them. Elsewhere, Jinbe and Shirahoshi ride Megalo to the plaza, seemingly having convinced Luffy to stand down.
As King Neptune's execution draws near, Jinbe and Shirahoshi are also captured. The king and the princes hear a shocking new detail regarding the assassination of Queen Otohime 10 years ago. As Hody begin his assault, an unexpected turn.
The battle between the Straw Hats and Hody's forces begins. Luffy instantly defeats 50,000 of his soldiers, and as the Straw Hats begin to make quick work of the remaining 50,000, Luffy charges forward at Hody himself.
As the battle between the Straw Hats and the New Fish-Man Pirates unfolds, an ancient ship called Noah is seen falling onto Fish-Man Island. The island's minutes seem numbered, but Shirahoshi makes a desperate break to save the island.
Decken faints and Noah falls back toward Fish-Man Island. An evacuation order is issued, and the entire island falls into chaos. Fukaboshi reveals the truth about Hody and the New Fish-Man Pirates while Luffy launches an all-out attack.
With the threat of Hody and the New Fish-Man Pirates behind them, Luffy and the Straw Hats have one more promise to keep. Luffy tries his best to destroy Noah, but at the last minute, the fate of the island depends on some unexpected help.
