One night while docked near an island, all of the crew starts to fall asleep, with only Robin staying up reading a book, and a young man blows into a seahorse-shaped instruments underwater. That morning, Robin greets a waking Nami only to find that she cannot recognize her. In the boys' bedroom, Chopper was the first to wake and ran away when he saw that he was surrounded by humans. When the other men wake up, they all try to figure out what is going on since they have no idea how they got there or who everyone is, with the last things they remember is events right before Luffy arrived in their lives (In Luffy's case, before he found himself on Alvida's ship), and Usopp tells them that they are all part of his crew and lost their memory in a storm fighting a giant whale. However, Sanji quickly sees through this lie. He decides to make breakfast for everyone so they head up to the deck, and when Robin comes up with Nami to say that Nami lost her memory, Sanji falls in love all over again. They soon see Chopper hiding behind a barrel, having no idea what he is, so Luffy starts chasing after him. Robin says that they are on a pirate ship and everyone freaks out, especially Nami who still hates pirates without her memories of the crew. At this point, Zoro jumps down from the crow's nest and Usopp freaks out when he realizes that he is the infamous bounty hunter. Nami had gone back into her room and found a picture of her, Bell-mère and Nojiko, and rushes off the ship using the Waver while bringing along the picture and all of the gold from Skypiea. With this, Zoro decides that he has no reason to stay either and goes to the island as well.
Nami comes up to the island into town to find everyone fighting over a newspaper, which serves as their only source of information since they lost their memories too. Nami tries to find a ship she can use to get back to the Cocoyashi Village but the boy from the last night appears and explains that all of the boats are wrecked. In the dining room, Robin tells everyone left what she remembers of their adventures, but being the latest member, she does not know the circumstances under which everyone joined. She then recounts what happened the previous day and says that she heard a strange sound. With a whistle from the tea kettle, everyone with memory loss realizes that they all had the same dream about a boy underwater blowing into an instrument, and only Robin was not affected because she was awake. They reach the conclusion that the boy is the cause of the memory loss and that he may strike again that night, possibly taking all of their memories this time.
That night, the boy indeed strikes again and is surprised to find everyone awake. Luffy attacks him, damaging his seahorse horn and all of his memories return, but the boy escapes. Luffy tells everyone their roles, but wants to bring back Zoro and Nami, though Robin convinces him to wait until morning. The next day, they come onto the island and immediately bump into Nami, and Luffy forces her to help them find Zoro. On their way, Robin comes across some ancient writing and, realizing the true cause behind the memory loss, has everyone go to the "Sea God's Palace". Meanwhile, the boy hypnotizes Zoro into joining his side.
As the crew journey into the forest, Usopp and Chopper are separated from the group and encounter Zoro. They bring him back to everyone else but he suddenly starts to attack Luffy. Chopper realizes that he's been hypnotized and Luffy tells everyone to go on ahead without him. Farther down the path, Nami tries to convince everyone to stop following Robin and join her instead, since she can help them get back to their villages, but they all decline and Nami decides to follow Robin for now to get back the bag of gold that Sanji volunteered to carry for her. They reach a lake and find the boy, who tries to hypnotize everyone, but all of their eyes are either covered or closed and Sanji kicks the seahorse out of his arms. The boy runs away, but Robin explains that he was just hypnotized by the seahorse. Sanji starts kicking the seahorse around and eventually the entire crew gets back their memories. However, Zoro comes out of the forest to the lake without Luffy. When he does not respond, Sanji attacks him, but it turns out he also regained his memory and Luffy was just right behind him.
The seahorse then reveals his ultimate technique. He first makes everyone see their loved ones that were left behind (Nami- Bell-mère ; Chopper- Dr. Hiriluk; Usopp- Kaya; Zoro- Kuina; Sanji- Zeff; Luffy- Ace and Shanks; Robin's mind was never drained of her memories, so the seahorse did not know about her loved ones) and makes everyone see their enemies (Arlong, Wapol, Kuro, Don Krieg, Crocodile, Enel) making them vulnerable, then takes every bit of their memory. As he does so, he becomes increasingly obese and starts floating in the air, claiming he's now a Sennenryu (Millennial Dragon from Warship Island Arc) like he has always wanted. Despite having lost all of his memories, Luffy decides that he wants to ride the seahorse and unintentionally uses his Devil Fruit powers to fly into it and wrap his arms around it. This causes the seahorse to return Luffy's memories, so he beats up the seahorse until everyone's memory returns and sends it flying off the island.
As the crew comes down to the village thinking they will be rewarded for getting everyone's memories back, they are instead attacked. Dream, the boy who carried the seahorse around, misinterpreted the situation and thought the Straw Hat Pirates were the ones who took everyone's memories and warned the entire village, so the crew is forced to rush back to the ship and leave.
- One Piece Anime — Episode 220, The crew lost their memories.
- One Piece Anime — Episode 221, Nami is confused by the villagers' behavior.
- One Piece Anime — Episode 221, Luffy regains his memories.
- One Piece Anime — Episode 223, The rest of the crew regain their memories.
- One Piece Anime — Episode 224, The crew's epic escape.
Ocean's Dream Arc