One Piece: Giant Mecha Soldier of Karakuri Castle

The Straw Hats find a chest hiding an old woman with golden teeth. She tells them about a mechanical island and a legendary treasure, which the crew, especially Nami, cannot resist, so they decide to set off on an adventure. Once on the island, they encounter Ratchet, the old woman's son, who, believing the pirates have kidnapped his mother, attacks the Straw Hats. Once the misunderstanding is cleared up, they agree to team up to find the treasure.