Anyone who has paid attention to Nami's backstory knows that she used to be a cat burglar and her past comes to haunt her in this movie. Coincidentally, this movie is tied into the Heart of Gold anime episodes and Gold movie story lines. As always, the Straw Hats band together and dispatch the baddies before moving on to their next adventure.
This movie features a nut-ball of a pirate captain as the bad guy. He has a huge crush on Nami and wants to use her for her navigational skills. Also, the guy controls gravity that he can target like magnetism... So there is that. As in typical One Piece fashion, they guy is a jerk and has to be shut down.
This is the first time that I have seen an anime movie with a story set ahead of the series. The series began airing a few weeks later and I was confused as to why some characters did not seem to know the others. Other than that, it is an action-packed romp with the children of the original Naruto cast.
First, while not entirely necessary to watch, the characters in this movie are the motive behind events in the television movie named, "Heart of Gold". Second, this movie is long and it drags a little bit. However, there is a lot of great action scenes and it falls into the post-Flamingo timeline of the series.