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.
