One Piece: Mega Mecha Soldier of Karakuri Castle

Luffy and his crew find a treasure chest on an abandoned pirate ship. Inside they find an old lady who says she'll give them a golden crown if they take her back to her island. When they reach the island, they hear a fisherman singing the "Yurari Song", which mentions the golden crown and contains puzzling hints as to how they can find it. Then they are attacked by Ratchet, the old lady's son and the lord of the island, since he thinks his mother is being held hostage by the pirates. Ratchet is also trying to figure out the Yurari Song and decides to use the Straw Hat Pirates. (official distributor synopsis)