Based on

Movies like One Piece "3D2Y": Overcome Ace's Death! Luffy's Vow to his Friends (2014)

The special takes place during the two year before the Straw Hats reunite on Sabody. Luffy is currently in Rusukaina training to get stronger to take on the New World. However the training is interrupted when Hancock's sisters, Marigold and Sandersonia, are kidnapped by the Byrnndi World, a pirate who was locked away on Level 6 of Impel Down but escaped during Luffy's invasion to save Ace, in order to lure Hancock to him and use her as a hostage against the World Government due to her Shichibukai status. Thus Luffy and Hancock head off to confront him and save Hancock's sisters.