Ocean's Dream Arc

The anime's sixth original storyline draws its premise from a PlayStation title, the game Ocean's Dream, rather than from any manga material. Its hook is memory theft: robbed of their pasts, most of the crew mentally revert to the moment just before Luffy recruited them, and old fears and hostilities come flooding back. Robin, being the most recent to join and the only one still conscious when the horn sounds, keeps her mind intact and becomes the group's anchor as the others try to rebuild their identities and their trust in one another.

As the crew dozes at anchor near an island, a young man beneath the surface pipes a seahorse-shaped instrument that quietly siphons their memories. Come morning, the men no longer know one another, remembering only what happened before Luffy first entered their lives, while Robin, who spent the night reading, is left unaffected. Usopp's bluff about being the captain collapses at once, and fear takes over: Nami, once again loathing pirates, bolts from the ship with the gold hauled back from Skypiea, which prompts Zoro to walk off as well. Robin deduces that a shared dream of a boy with a horn lies behind the amnesia, and that he is likely to return after dark.

The boy does strike again, but finding everyone awake this time, he is caught off guard; Luffy cracks his horn, recovers his own memories, and the culprit flees after hypnotizing Zoro into turning on the crew. Guided by ancient inscriptions Robin has read, the group heads for the so-called Sea God's Palace. Following more clashes and a struggle at a lakeside, they wrestle their memories back, only for the seahorse to loose its final trick, forcing each of them to relive their lost loved ones and old enemies before draining every last thought and bloating into the dragon-like shape it has always longed to be. Even wiped clean, Luffy reflexively latches onto the drifting creature, gets his memories returned in the process, and beats it senseless until the whole crew is whole again, hurling it off the island.

Because it sits outside the canon, the arc changes nothing in the ongoing story. Expecting gratitude from the townsfolk for restoring everyone's memories, the crew is instead set upon, since the boy who ferried the seahorse, Drim, has wrongly told the village that the Straw Hats were the memory thieves. With no time to explain, the pirates scramble back to their vessel and put the island behind them, their recollections and their bonds fully restored.