After a young woman named Tess commits suicide by leaping from the penthouse apartment of Herbert Folsom, the policeman investigating the case finds that Folsom has been shot. With his dying breath, Folsom utters the name "Casey." The case is turned over to Captain McGowan, who discovers that Folsom was a wastrel who would not divorce his wife despite his open philandering. When Arthur "Casey" Jones, Tess's brother, is found in Lois Martin's apartment, which is located just below Folsom's, Casey admits that he went to Folsom to demand that he marry Tess, whom he had recently discarded. Casey insists, however, that although his heart was set on vengeance and he carried a pistol, he found Folsom already shot. Another suspect extinguishes the lights during the interrogation, and engineer Larson is sent for. Larson is about to tell what he knows about the murder when the lights are turned out again and he is shot. When the lights are turned on, McGowan finds the murder weapon in the hands of Folsom's pet ape, Garcon, who was given it by the killer. Deciding that Casey must be the culprit, McGowan tells him to remain behind after the others are dismissed. Lois believes that Casey is innocent, however, and begins her own investigation. Meanwhile, the apartment building's nervous telephone operator, Bella, is forced by the unusual events to cancel her dancing date with her boyfriend Elmer. Suspecting Folsom's friend, Lewis Leroy, Lois snoops around until she ends up in Larson's basement apartment. There she finds various mechanical devices and a card with numbers written on it. After discovering the clues, Lois returns to Folsom's apartment, where she is surprised by Leroy. She tricks him into leaving the room for a few moments, during which she gets McGowan on the telephone so that he can listen to what transpires. Lois then reveals that Leroy killed Folsom because he hoped to marry Sylvia, Folsom's wife. Leroy hired Larson to fit Folsom's radio with a gun that would fire blanks when the radio was turned to station KCA. Unknown to Larson, Leroy loaded the gun with real bullets, and Folsom was killed when he turned to KCA to listen to a speech given by Leroy. Folsom's last word was therefore "KCA," and not "Casey." Leroy then killed Larson out of fear that he would reveal the scheme. McGowan arrives, but as he is about to arrest Leroy, Leroy turns the radio to KCA and kills himself. With the case solved, Lois is free to pursue her interest in Casey, whom she liked at first sight.
