Dream is encountered on an island in the Grand Line where people have had their memories stolen.
The crew of the Going Merry have had their memories stolen and this boy is initially believed to be the culprit. It isn't too long before Nico Robin finds out true identity of the memory thief. The boy has been possessed by the real thief, but is soon set free when Sanji separates the boy from the sea horse horn that he's been holding since his first appearance (the horn is the memory eater and lives by eating memories). After a good beating the memories of everyone on the island are vomited up by the horn.