Story 4 is The Tell-Tale Film. The premise is that Lupin watches a movie called "The Happy Princess", and he's so bored by it that he starts focusing on random background characters. He notes that the man playing the butler is clearly in love with the female lead. In the second part of the film, the same actor plays a crazed woodsman, who kidnaps the happy princess. Lupin suspects the actor wanted to kidnap her in real life.
On a hunch, he checks with the studios. The woman playing the princess disappeared recently, when scheduled to go to America. At the same time, the actor stole a car and kidnapped an unknown woman. Lupin believes he kidnapped the actress.
Our heroes go to the place where the movie was filmed, and what's this? The actor is posing as a real-life woodsman! Lupin spends a few days trailing him, and although the man makes a secret trip to his hostage every night, Lupin can't figure out where he's going or where she's located. (This turns out to be one of those mysteries that you can't figure out, unless you're familiar with French geography. She's on the other side of the Seine, in the same relative area.)
Lupin confronts the actor, to no avail. Then the police arrive, and Lupin tries to sneak the actor past the police. The plan doesn't work, and he's left with a rusty saucepan that the actor was hiding.
Lupin realizes the saucepan is used to bail water out of a small boat. He uses it to cross the Seine, and he finds the actress in the cabin from the movie. She wasn't kidnapped at all! She and the actor are madly in love, and they're happily reenacting the movie. The entire kidnapping was a misunderstanding!
In a dark twist, Lupin insists that she was originally kidnapped, but it only took a few hours for her to go crazy and develop Stockholm Syndrome. Lupin's lady friend feels bad for them, so Lupin does her a favor and helps the two actors flee the country.
I did not like this story at all. "Someone finds a mystery in a movie" is a good premise, but I didn't like the way it played out, with them reenacting the movie at the exact same location the movie was filmed, without any good reason why. I further didn't like how the victim went insane, but Lupin helped the couple escape and live happily ever after. No. He should have been arrested, and she should have been rehabilitated.