One of the classic Animes.
This movies was release in 1993 and has had a big influence on anime films that followed. The other anime's that also shaped the direction of anime to a similar extent are Akira and Ghost in the Shell. These three films showcased anime to western audiences.
The plot isn't super complex, we have the lone hero who saves the girl and thwarts a plot to overthrow the current leaders of the country. This is of course way, way oversimplification of the movie.
Slightly more detail, we have Jubei Kibagami (our lone hero), he fights a total of eight demon leaders with other ninja followers. They want to rule Japan, hence they are plotting to overthrow the current Shogun.
No hero can do everything himself, and Jubei is aided by a government official (Dakuan) who keeps his distance initially. The girl (Kagero) he comes across has a secret, which is that here body has poisons running through it.
If you haven't seen this anime and are interested in Japanese anime, you should watch this one.