Whose footprint is found next to the body?
So Dark the Night is the story of a detective, Henri Cassin from Paris, who, while on a long overdue vacation, falls in love with innkeeper Pierre Michaud's daughter Nanette. She is a country girl with a jealous boyfriend. Nonetheless, the detective becomes engaged to her. Then, the night of her engagement party, the girl vanishes and later turns up dead. Cassin believes that the obvious suspect is Leon, the old boyfriend, but soon he is also found killed. Soon after Mama receives a warning that she will be the next to die, she is found strangled.
Pierre, fearing for his safety, decides to sell the inn. Henri returns to Paris and, because of his investigative skill, he is able to come up with a sketch of the killer by expanding and elaborating on information provided by a footprint found beside Leon's body. 
To Henri's astonishment, the sketch bears the exact likeness of himself, and when he fits his shoe into the footprint, he realizes that he is undoubtedly the killer. After making a full confession to the police commissioner, Henri is evaluated by a psychiatrist, who determines that he is schizophrenic. Though placed under watch of a guard, Henri escapes back to St. Margot, where he tries to strangle Pierre. However, the police commissioner, who has followed Henri to the village, catches the detective in the act and shoots him.
Henri's