With Usopp now in the crew and a new ship, the Going Merry, under their command, the Straw Hat Pirates head to an island said to hold a fabled treasure, only to encounter a strange man stuck in a treasure box named Gaimon that has also been looking for this treasure.
While at sea, Usopp daydreams, unbeknownst to the other crew members. Since the crew has increased to four, Luffy wants to make a jolly roger. With "50 years experience" of painting walls, Usopp creates one for the Straw Hat Pirates. He also paints the sails as ordered by Luffy.
Their first destination is Legendary Treasure Island. There, they encounter strange looking wild animals. 'Get Out of Here! Get Out of Here!' the Guardian God of the island commands. The Guardian God attacks the Straw Hat Pirates. When all the attacks fail to hurt Luffy, the Guardian God runs away. Luffy catches him and finds out that he is actually just a man living in a box, named Gaimon.
Twenty years ago, he was a pirate who fell into the box and was stuck ever since. Luffy and Zoro futilely try to pull him out of the box but their efforts were in vain. Luffy introduces himself as the one who is going to be the Pirate King after getting One Piece. Gaimon fills Luffy in on what happened to him 20 years ago: how he fell into the box.
Gaimon takes the pirates to the hill where there is a treasure when Nami promises to get it for him. Luffy rockets up the hill and finds five treasure boxes but he refuses to hand them over to Gaimon. Nami thinks he's keeping them for himself, but Gaimon already knew/felt that the boxes were empty.