Eloise, the daughter of d'Artagnan, is in a convent when a runaway slave appears, looking for safety. He doesn't find it, as his pursuers arrive and kill the mother superior. Eloise finds a piece of paper that the slave has used to cover a wound and believes that it's the coded key to a conspiracy (it's a laundry list). She's a spirited young woman, so she goes off to find her father, who, she's sure, will break the conspiracy and save the king.
The movie's a comedy, by the way. There's lots of stuff about the supposed code, and the evil Cardinal Rochefort is involved in so many conspiracies, he's perfectly willing to believe that he's involved in this one. Or another one, since a poet in love with Eloise has written a poem ("Dance, butterfly, dance") that's fallen into the cardinal's hands. And, sure enough, there's a real conspiracy, as the evil Duke Crassac and his beautiful (but evil) paramour are out to kill the king.
There are lots of sword fights, and while Eloise isn't skilled, she's very enthusiastic. The Musketeers, having grown old, aren't enthusiastic, but they're still skilled enough to defeat any number of villains.
You probably have to be in the right mood for this one. It's the kind of movie in which a man believed to have been dead for many years turns out to be alive. "I can explain," he says, but he never does. I thought it was funny, and I enjoyed the geezerly action. Check it out.