John Quincannon is a private detective in San Francisco whose current client is named Titus Willard. After observing a covert meeting between Willard and a man who is blackmailing him, Quincannon follows the blackmailing man from a bar parlor. The man enters a cigar shop down Gunpowder Alley with the name plate "R. Sonderberg." Next door to the shop, an elderly woman sits, observing the street. As Quincannon is preparing to investigate the shop further, he sees a policeman walking down the street. The policeman introduces himself as Maguire, and as the men are talking, they hear two loud pistol shots. When the men break into R. Sonderberg's shop, they see the blackmailer dead on the floor, the gun splayed in front of him. Maguire declares it a suicide, as there's no possible way somebody could have gotten in and out of the bolted-up building so quickly. Quincannon knows better; the shots on R. Sonderberg's body are not in a common place for a suicide, and the gun is too far away. He does a quick search of the place and notes that the $5,000 in blackmail that R. Sonderberg extracted from Titus Willard is nowhere to be found. Before he leaves the area, Quincannon knocks on the door of the old woman next door, who tells him she saw nothing, but heard the shots. The next day, Quincannon talks with his partner and unrequited love Sabina about the case, and they discuss possibilities. They decide that Quincannon should have another chat with the old woman, but when he returns to the house, there's nobody home. He confirms with nearby residents two important facts: that Sonderberg frequented variety houses, and that the house next door to his shop, where the elderly woman was, has been empty for months. He knows who the murderer is and how they got away with the murder. Quincannon visits Titus Willard and forces him to tell him why he's being blackmailed. It's what Quincannon expected — an illicit affair with an actress named Pauline Dupree. Quincannon and Willard pay a visit to Pauline at the gaudy theater where she works, and Quincannon lays out his theory: that Pauline was R. Sonderberg's co-conspirator the whole time, that she was also the elderly woman, disguised in a wig and makeup, that she pulled off the murder quickly and easily by pretending to live in the house next door. Now, she has the ten thousand dollars all to herself. Willard refuses to believe this, defending Pauline, and Quincannon eventually leaves in defeat with no tangible proof. On his way out, Pauline gives him an exaggerated wink. Quincannon despairs of the experience to Sabina, who says the important thing is that he solved the case. Quincannon ask Sabina what kind of man makes such a fool of himself over a woman, and Sabina looks at him pointedly, and tells him, "All kinds, John."