ONE PIECE SPECIAL: THE DETECTIVE MEMOIRS OF CHIEF STRAW HAT LUFFY

Unlike the other specials, this story takes place in an alternate reality version of 19th century Japan, in "The Grand Jipangu", an alternate version of The Grand Line as opposed to simply being hour-long filler episodes.The story is divided into two halves: in the first, Buggy the Clown makes trouble in the town where Detective Luffy is stationed, and in the second a mysterious girl named Vivi appears. These stories feature cameos from several minor characters in the normal series. Surprisingly the "Oyabun" (Boss) specials did not stop there as five more specials were made following the first one, mostly as Christmas and New Years specials as well as fillers once in awhile. — Description from AniList community (anilist.co).