This takes place during the Alabasta Arc. when the StrawHat Pirates help out Vivi (a valiant princess)save her kingdom from the Baroque Works' conspiracy plot. The Kingdom of Alabasta is a dessert island that hasn't had a drop of rain for 3 years. King Cobra(Vivi's father) tried every he could to help his people and kingdom from falling. After all the unsuccessful promises on rain. Most of the locals and villagers plan to start a war against the kingdom itself. But little do they know is that the whole thing is a setup made possible for them all to kill each other.
Those who seen the Alabasta episodes would be familiar with the whole story and what happens. Only difference is that this movie only shows you the important events and side stories in an edit way.
You might ask what is so great about watching a movie that retells a story we already seen from the TV series. Well I'll tell you why. This movie is an alternate retelling of the Alabasta episodes. When it comes to alternate you see events and small elements that weren't in the TV episodes.
Such as different animation quality since it's a cinema film. The flow of the story is slightly different. The battles are much more massive. The drama has a bit more suspense. The background and locations are richer with the use of CGI. And the fights and visual effects are more complex. All of this makes the whole Alabasta story entirely different from what we've seen before on TV.
Another good reason in seeing this film is those who miss Vivi will get to see her again in this movie. And those who like to see Luffy fight Crocodile will be able to see it again in an entirely different view.
Overrall, Toei Animation did a great job in alternating this story arc. It be a masterpiece if it included more events and characters that were left out. If that happened it be a much longer movie. To all One Piece fans, this movie will be released by Funimation in 2008. Be sure to check it out sub or dub by then.