Mystery #6 is The Lady with the Hatchet, which seems to be based on the story of Jack the Ripper. A serial killer is loose in Paris. Every time, the killer kidnaps a woman, and a week later, the woman is found tied up, half-starved and dead from an ax wound to the center of her forehead.
The killer's notebook is found, with a list of victims and days, indicating that victim #7 will be taken soon. Everyone goes over it, but all they can figure out from the handwriting is that the culprit is a well educated woman.
This time, the killer makes the mistake of kidnapping Arsene Lupin's lady friend. Lupin vows to save her! He spends three full days in his room, reading the newspaper accounts of the murders and trying to figure out the connection between the victims. When he figures it out, he puts an ad in the paper, goes through the three responses, and heads off the stop the killer.
The story had me, until the point where it revealed the connection between the victims. They are all women with eight-letter names, beginning with "H", like Hermione, Hortense, Honorine, and Hilairie. Nobody figured this out until just now, because the newspapers referred to the married victims as "Mrs. So-and-So", omitting their first names. Only two of the first names were publicly released.
I find it really, REALLY hard to believe that the police investigating the serial killer did not notice all the victims' names start with the same letter.
Well, eight-letter names that start with "H" are not common, so Lupin puts an ad in the paper, pretending to be a woman named Herminie. The killer takes the bait, and Lupin finds himself at the home of Mr. Lourtier-Vaneau, a happily-married man. After a good deal of pressing, the man reluctantly admits that his first wife isn't dead, like he claims she is. She's at an insane asylum, having been committed shortly after the death of her two children. She is the serial killer, and she kills herself when Lupin breaks into her secret hideout to save his lady friend.
His lady friend is unaware that she was a captive of the serial killer. She thought she was the captive of a random crazy woman. And around this time, her husband dies, meaning she is now single and able to pursue a romance with Lupin.
I liked this story at first. It was a pretty serious change of pace and tone, to jump to a heavy serial killer story. The way in which he deals with Mr. Lourtier-Vaneau is dramatic and exciting, as is the scene where they break into the secret hideout. It was also a minor mystery/twist to learn that Mr. L-V's wife is still alive. But I cannot get over the fact that nobody noticed all the victims have similar names. That's ridiculous. Even if the newspapers only revealed two of the first names, someone should have suggested that a long time ago, especially since they were competent enough to realize when every murder would take place.