Boss Luffy Historical Special

The Boss Luffy Historical Special is a stand-alone anime-only special that began airing on December 18, 2006. It is set in an alternate universe, and uses many different One Piece characters to tell its story.

Summary
In an alternate universe set in Edo period Japan, the Mikoshi (portable shrine) race is about to take place, the winner of which receives 1,000,000, and Boss Luffy and his friends recruit Franky to build them a mikoshi. Meanwhile, Thriller LTD enters the race, and begins working to sabotage its competitors, including groups based on the Flying Fish Riders and Rolling Pirates. The crew meets up with Brook, who cannot remember anything other than his own name, and tries to stop Thriller LTD from interfering. They arrive to find their mikoshi destroyed, and Brook remembers that he was once part of Thriller LTD before trying to leave and being thrown in a river by Hogback, Perona and Absalom.

Robin reveals Brook's past to Luffy and his friends, and they decide to help him. Brook is captured by Thriller LTD, but Luffy and Zoro arrive to help him escape. The group rebuilds their mikoshi (portable shrine), which is modeled after the Thousand Sunny, and faces Thriller LTD's Oars-shaped mikoshi in the race. They win with the help of its special abilities, and Luffy sends Hogback, Perona and Absalom flying for disrupting the festival. Igaram confiscates the prize money to pay for the damage to the town, but allows them to see the treasure, which is a pair of panties that can grant wishes if a beautiful woman wears them. Brook suggests that Mao wear them and she punches him out, once again erasing his memory.

Trivia

 * This is the only filler arc that does not fit into the continuity of the One Piece story.