Warning: Wikipedia contains spoilers!
Probably the definitive Lupin III movie, Castle of Cagliostro was written and directed by Hayao Miyazaki before he formed the famous Studio Ghibli. The movie is filled with adventure, romance and subtle nuances of emotion. There is a longstanding rumor in American anime circles that Steven Spielberg considered Castle of Cagliostro "the best action movie ever made." The movie is based on gentleman thief Lupin III, grandson to French master thief Arsene Lupin.
The movie starts out with Lupin and cohort Jigen doing what they do best, escaping pursuit after having robbed the national casino of Monaco - only to discover that their entire haul is counterfeit. The bills are of amazingly high quality and could be none other than the legendary 'goat bills' - perfect counterfeits that have been used to rock the economies of nations since the invention of paper money.
When Lupin was just getting started as a professional thief, he almost was killed while searching for the source of the goat bills. He decides that it is time to take another chance, and the two head off to the (fictional) Duchy of Cagliostro - a tiny country (the smallest country in the United Nations according to Jigen) that appears to be a conglomeration of various old-world European locales.
Shortly after arriving, they rescue a young girl from a car full of thugs, only to let her get captured again when Lupin is knocked unconscious after tumbling down a cliff. They later discover that she is the late grand duke's daughter and is engaged to be married to the (obligatorially evil) count.
The count wants to recover the ancient treasure of the Cagliostro family, and needs the princess's ring in order to find it. Wackiness, chases and intrigue ensue.
Lupin's former lover, sometimes enemy/friend Fujiko shows up as an employee in the castle, cohort Goemon Ishikawa (a ninja sword saint) is called in to help the gang in their final assault on the castle, and Lupin arch-rival Interpol inspector Zenigata even finds himself allied with Lupin and company in order to expose the count.
Most of Lupin III's cast refers back to older popular Japanese entertainment. His nemesis Zenigata refers to a long running drama Zenigata Kagin - who is a 18th century Japanese Lawman who uses a jitte and throwing coins. Maurice Leblanc 's Arsene Lupin was a cousin to Sherlock Holmes.