Based on

Movies like One Piece Episode of Nami: Tears of a Navigator and the Bonds of Friends (2012)

After Nami steals the Going Merry, Roronoa Zoro, Usopp, and later Monkey D. Luffy and Sanji (along with Johnny and Yosaku) set after her and wind up in Cocoyasi Village, Nami's hometown ruled by the tyrannical fishman Arlong. It is here that Nami's past and true motives come to light.