First note that Rupan III and Lupin the 3rd are the same character and this is the same movie series. There was some legal trouble back in the 90s over the use of the 'Lupin' name, then still copyrighted in France, and so a couple of the movies were translated as 'Rupan'. So there.
Anyway, this time around the Lupin gang are in rural Japan because Goemon is getting married! But the bride's family are caught up in a feud with the mysterious and deadly Fuman clan, who are searching for a vast treasure in gold. Lupin promises to get to the gold and steal it first, but discovers that it is hidden in a tricky labyrinth - obviously inspired by the Indiana Jones movies, which came out just a few years earlier. The traps and dangers are very cool and imaginative, as is a breakneck car chase scene through a hotel that occurs near the beginning of the picture. I saw this movie in 1992 as a fansub version and it immediately became one of my favorite anime features. The action animation, comedy timing, and suspense are top notch and even though the run time seems a bit short at 73 minutes, that is simply due to the breakneck pacing of the thing and not a scarcity of plot. A lot happens and the whole thing hangs together better and entertains a heck of a lot more than Indy IV did.