In 17th century kingdom of France, Henri (Jean-François Poron), a mysterious prisoner wearing an iron mask, escapes from the isle of Sainte-Marguerite. It turns out he is the hidden twin brother of King Louis XIV (also played by Jean-François Poron), and a bunch of conspirators want to use him to replace the king and take over the kingdom. But Henri wants none of it and only wishes to escape as far as possible with Isabelle de Saint-Mars (Claudine Auger), the woman he loves. With her help, he manages to pull it off.
Enters D'Artagnan (Jean Marais), who was tasked by the Cardinal of Mazarin (played by Enrico Maria Salerno) to bring the Iron Mask to Paris. He must now solve this mess.
The Iron Mask provides examples of:
- Actor Allusion: At one point, D'Artagnan disguises himself as a hunchback to avoid being chased by maréchaussée soldiers. Jean Marais previously played a nobleman disguising himself as a hunchback in Le Bossu.
- Adapted Out: The Three Musketeers — Athos, Porthos and Aramis — are nowhere to be seen.
- And the Adventure Continues: At the end, just when D'Artagnan earns a well deserved private moment with his love, Madame de Chaulmes, he gets a new assignment yet again, and leaves again.
- Badass Mustache: D'Artagnan sports a mustache, and has no match when it comes to fencing.
- Boisterous Bruiser: D'Artagnan brags quite hammily, and kicks ass.
- Comedic Spanking: D'Artagnan spanks Isabelle de Saint-Mars to punish her for lying about Henri to him in order to cover the latter's escape.
- Fainting: Happens to Isabelle when Henri is being shot at on a roof. Then it happens again when she sees King Louis XIV's face, as he is Henri's hidden twin brother.
- Large Ham: Jean Marais had the time of his life hamming it up when playing D'Artagnan.
- Lighter and Softer: Without ever veering into parody, the film had significantly more humor than most French swashbuckler films of the time (including those based on Alexandre Dumas' D'Artagnan Romances and Jean Marais' own previous films in the genre), and much less people (if any) get skewered by swords in it. It is perhaps the most lighthearted Man in the Iron Mask film overall as well.
- Man in the Iron Mask: One of the most faithful elements to the story from Le Vicomte de Bragelonne in that the prisoner with the iron mask is Louis XIV's hidden twin brother.
- Master Swordsman: D'Artagnan, of course.
- Not Even Bothering with the Accent: It's safe to say that Jean Marais' few attempts at a Gascon accent didn't work out that well.
- Old Soldier: D'Artagnan is quite far from the days when he met the Three Musketeers by this point (Jean Marais was 49 at the time). But he's also far from being out of shape.
- Running Gag: D'Artagnan badly wants intimate moments with Madame de Chaulmes. He keeps getting interrupted or getting called upon for a mission whenever he has such moments.