Source taken from Wikipedia.org
One Piece (Wan Pīsu?) is the first animated feature film of the franchise, starring Mayumi Tanaka as Monkey D. Luffy, Kazuya Nakai as Roronoa Zoro, Akemi Okamura as Nami, and Kappei Yamaguchi as Usopp. It premiered in Japan on March 4, 2000 and was released to DVD on January 21, 2001. The film was shown in a double bill with Digimon Adventure: Our War Game!. In their first week of showing, they made a second place in the Japanese box office, a third place during their second week, and two first places during their fourth and fifth weeks. In total, they earned 2,160,000,000 Japanese yen.
The legend of the Great Gold Pirate Wonan remains intact, years after his disappearance. Many pirates search for his mountain of gold hidden on a remote island. Among them are Captain El Drago and his men. By hunting down every member of Wonan's former crew, they eventually take possession of a map to that hidden island. On their way there, they meet and decide to rob the Straw Hat Pirates, who, still lacking a cook, are close to starvation.