The Castle of Cagliostro. Okay, it's based on a series, but it's a standalone film, like all Lupin films and all you really need to know is that Lupin is a thief, Jigen is a marksman, Goemon is a samurai, Fujiko is also a thief and Zenigata works for Interpol and that they're all friends.
Well, except Zenigata. And Fujiko a lot of the time.
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Still the number one reason for the success of the Internet.