Watch One Piece: Baron Omatsuri and the Secret Island Online

In an alternate reality world that resembles Edo Period Japan, Luffy acts as a member of the police. This is made up of two adventures: in the first, Buggy makes trouble in the town; in the second, a mysterious girl named Vivi appears (aired after Episode 253). This special contains several cameos from characters throughout the entire series. A second pair of episodes were aired after episode 290, continuing this story, as part of the New Year's special.

For many years, Ex-Marine Lieutenant Randolph and his troupe have put on a play aboard his ship. Having lost his family to pirates, Randolph wishes to put on plays that will give courage to those who have also lost their loved ones to pirates. Unfortunately, Randolph must retire soon, and he is now giving his last performance. Luffy and his crew come to see the play, and somehow manage to take part in it themselves. But from Randolph's past comes a vengeful subordinate with a diabolical plan who has finally become commander in order to capture Randolph. Luffy and his nakama now must prove to the people that not all pirates are scum, and protect Randolph's final farewell performance from the bitter commander. (aired after Episode 174) (Source: ANN)

The story opens on Pirate Zap's ship, where two of his crew, Bonnie and Max, are tired and want to escape, but unfortunately they have no money. Three children were being held captive on the ship overhear them. The eldest, Amanda, who's father was a pro treasure hunter, knows the whereabouts of a great treasure, and offers them a deal. If they help them make a clean escape, they could take all the treasure they wanted. They agree, and the five of them barely escape and make it onto a small island where they meet Luffy and his crew. Unfortunately they were pursued and Luffy and Amanda are captured and brought back to their boss, the head of the Bayan Pirates, who is also after the treasure. Now Luffy and the others must battle the Bayan pirates and find the treasure that Amanda's father had left for his children. Amanda, who has always resented adventure and treasure because her father was constantly gone in search for it, finally understands his feelings. (Source: ANN)

The Straw Hats encounter a city in the middle of a whirlpool, called the Ocean's Navel, that is being destroyed by giant monsters. These monsters supposedly protect a treasure capable of granting wishes, but in the process of defeating these guardians, the crew ends up releasing another evil. (aired after Episode 53) (Source: Wikipedia)

A recap OVA summarizing the ten-year TV series up to the Impel Down arc.