Story 5 is Therese and Germaine. At the end of the third story, Lupin overheard a brother and sister talking about murder. They're going to murder a married person on the cliffs, at a particular town, at a particular time.
(So convenient for Lupin that he overheard a murder plot in a murder mystery book! Also, doubly convenient that he only heard part of the plot, so there could be a mystery as to who the victim is!)
The only married couple seems to be the d'Ormevals, a nice couple. He loses his key by the bottom of the cliffs. He and his wife search for it. Afterwards, he walks to his cabin, waving off some bridge players as he does so, and goes inside. His wife spends time on the balcony for a while, before leaving.
After this, Mr. d'Ormeval is found dead in his cabin, stabbed in the back. But how could someone stab him, in a locked room? Is this related to the murder plot we learned about earlier?
Lupin finds the brother and sister at the cliffs, right on cue. He takes them to the crime scene, and a huge fight breaks out between the widow (Therese) and the sister (Germaine).
It is revealed that Germaine is Mr. d'Ormeval's mistress, and she planned to kill Therese on the cliffs, so she could have the husband all to herself. Germaine learned about the plot through some letters, and in a moment of passion, she stabbed her husband. He still loved his wife and didn't want her to be arrested for murder, so he put on a jacket to cover the knife, walked into his room and locked the door, in an attempt to exonerate her.
This story was pretty good, with nice dramatic twists in the extended explanation scene, like "the murder that happened wasn't the planned murderer", "the wife did it", "she's the mistress" and "the letters about the murder were stolen!". I just wish the setup for the mystery was better. Seriously, it's a bit of a stretch for a detective to accidentally overhear two people conspiring to commit murder.