A mysterious figure appears during the night and steals the memories of the crew. In the morning, Robin is the only one who remembers the Straw Hats adventures together. Her attempts to keep the crew together fail as Nami and Zoro go separate ways.

Robin suggests that only those who were sleeping lost their memories. Luffy and the others then remember seeing the mysterious boy who played some kind of flute. Later that night, they discover the boy outside and Luffy tries to fight him. At the last second, he proclaims his memory has returned.

During the night, Usopp, Chopper, and Sanji question what brought them to the Going Merry. In the morning, Usopp builds them a raft and they sail to the island where they meet Nami again. Meanwhile, the boy sets his sights on Zoro.

On their way to "The Palace of the Sea God" Robin's group meets an entranced Zoro. It looks like Zoro wants to fight so Luffy urges the others to keep going without him. They apprehend the kid but the whistle was what was really stealing their memories. Zoro returns, still entranced, but Luffy's nowhere to be seen.

Luffy returns and knocks some sense into Zoro. Cornered, the sea horse spreads illusions of the people in their memories to distract them. It steals their complete memories and claims that it has become a Sennenryu. Luffy defeats it and returns all the memories but the townspeople are convinced they are the memory stealers.