A dashing young thief robs a casino, blowing up police car tyres and sneaking out of windows, and manages (Ocean's Eleven notwithstanding) to escape with the money, only to find it counterfeit. Causing an immediate traffic jam by shovelling it out of his physics-defying car, he decides to track the money down to a little-known European kingdom. That's just the first five minutes of this terrific film, and the fun has hardly begun.
Cagliostro's Castle was my introduction to anime, and it's hard to imagine a better transition-- if this were live-action, it would be world-famous. The hero, Wolf (Lupin in Japanese, I believe), is a likeable rogue with impossible gadgets, a bizarre streak of chivalry and a determined policeman on his tail, and if that doesn't sound like a recipe for great fun I'm not sure what does. Throw in an eternally unruffled swordsman, a matter-of-fact sharpshooter, a refreshingly love-to-hate villain, a sassy lone agent and a beautiful 19-year-old damsel in distress, and you've got this amazing film, which mixes romance and action with comedy and doesn't waste a single minute.
One word of advice: get used to EITHER the subtitled OR the dubbed version, don't try to like both-- they seem to have been based on scripts at different stages of production, and as a consequence don't always match up even at crucial moments. If you speak fluent Japanese, fine.