A group of pirates lead by Alvida plunder a ship, only to find a barrel which contains a strange boy named Luffy. Luffy is on a mission to find the legendary treasure "One Piece" and he befriends the ship's cabin-boy, Coby, who actually wants to become a marine officer. Luffy defeats Alvida and the two journey on in search of crewmates.

Luffy and Coby arrive at Shell Island, a Marine base. They meet the bounty hunter Roronoa Zoro, held under arrest by the infamous marine, Captain Morgan, and his son Helmeppo.

Luffy convinces Zoro to join his crew, but they must first defeat Captain Morgan. After freeing the island from Morgan's oppressive rule, Coby is accepted as a marine while the other two continue on their journey.