The Straw Hat Pirates are asleep — all except Robin — when they share a strange dream: a cloaked young boy underwater, playing music on a sea horn. The next morning, every crew member wakes up with no memory of Robin or of being a pirate. They still remember their lives before joining the crew, but nothing after. Robin has to find a way to reunite the crew, restore what they have forgotten, and determine what caused the memory loss in the first place.