This takes place during the Punk Hazard (short) saga where Law causes Nami and Sanji to swap bodies. Hilarity ensues as Sanji and Nami, without realizing that Law is going to change them back, have sex with each other.
One Piece (ワンピース) is a Japanese shonen manga series written and illustrated by Eiichiro Oda. One Piece follows the adventures of Monkey D. Luffy, a 17-year-old boy who gains elastic abilities after inadvertently eating a mystical fruit, and his diverse crew of pirates, named the Straw Hats. Luffy explores the ocean in search of the world's ultimate treasure known as the One Piece and to become the next Pirate King.