Hoping to inspire her husband Jeff, an author of murder mysteries, to write a love story, Nancy Troy rents a basement apartment at 13 Gay Street in Greenwich Village. Although Eddie Turner, the owner of the building, informs his new tenants that the electricity in their apartment has not been turned on and advises them to come back later, Nancy insists in moving in that night. As Jeff expresses a sense of deja vu about the edifice, Nancy sees her old friend, Anne Carstairs, climb the stairs. Anne becomes flustered upon learning that she and Nancy are now neighbors, and later in the hallway, Anne and her husband Scott, along with Turner and Lingle, another tenant, ponder why the Troys would choose to live in the building that they are all forced to inhabit because of a shared dilemma. Later that night at a neighborhood restaurant, Jeff meets restaurant owner and neighbor Polly Franklin when she joins him at his table after Nancy leaves to make a phone call. In an adjoining phone booth, meanwhile, Nancy overhears a corpulent man call someone and order him to meet him at the basement apartment at 13 Gay Street. Apprehensive, Nancy returns to the table to report the conversation, after which Polly hurries home to her apartment at 13 Gay Street. When Nancy and Jeff return home after dinner, they hear the water in the bathtub draining and become startled when a candle that Nancy has set down begins to move. Upon investigating, Jeff discovers that Nancy had placed the candle on the back of a large turtle. After recognizing the turtle as Hickory, the mascot of a speakeasy that he used to frequent, Jeff realizes that his new domicile was the former speakeasy. Nancy and Jeff climb into bed after the moving men deliver their furniture, and are awakened the next morning when the police troop through their bedroom to investigate the report of a dead body in their garden. After Jeff and Nancy recognize the corpse as the corpulent caller from the restaurant, the other tenants send Turner to identify the body as belonging to Louis Kaufman, a former third-floor tenant. When Kaufman's apartment is discovered to be empty, Jeff decides to knock on Polly's door and invites himself in for a cup of coffee. After Jeff begins to question Polly about Kaufman, she spills her coffee and leaves the room to get a towel. On the table, Jeff sees a letter addressed to Andrew Bruhl at 507 12th Street. When an autopsy determines that Kaufman drowned in their bathtub, Nancy begins to despise her new home but Jeff is intrigued and determines to solve the murder. Later, they ask Turner for a shade to cover their front window, and after he offers them a large screen, Lingle reprimands him because the screen is etched with Bruhl's address. That night, the Troys are awakened when a man sneaks into their apartment and tries to steal the screen. Jeff chases the man away, then notices Bruhl's address on the screen. Jeff goes to the address the next day, but the woman who answers the door claims that Bruhl never lived there. Proceeding to the post office, Jeff learns that Bruhl has moved to a new address on Tilton Place. Admitted to the address on Tilton Place by Mrs. DeVoe, who claims to be Bruhl's landlady, the Troys introduce themselves as the Turners. After Mrs. DeVoe informs them that the portly Bruhl used the address only for mail delivery, receiving weekly letters from the Carstairs, Polly, Lingle and the Turners, the Troys deduce that Bruhl must be Kaufman, and was blackmailing the residents at 13 Gay Street. After leaving Tilton Place, Nancy voices her suspicion that Mrs. DeVoe is too affluent to be a landlady, and was directed by the murderer to relate the story of the cash-filled envelopes and to confuse the description of Bruhl and Kaufman. When the Troys repeat their theory at police headquarters, Inspector Hankins confirms that Kaufman worked for the Bruhl detective agency. Upon returning home to their apartment, the Troys find Anne, hysterical, waiting to talk to them. Jeff tricks the frantic woman into admitting that Bruhl was blackmailing the tenants in the building: Lingle for art forgery, Anne for being present at the murder of a wealthy playboy, and Polly for writing love letters that could jeopardize her custody of her son. When Jeff declares that Bruhl is still alive, Nancy realizes that her husband is setting a trap to force the murderer to return to kill Bruhl. That evening, Jeff pays a cab driver to speed away with Nancy to the safety of a hotel, but Nancy thwarts his plan by having the cab driver arrested and then returning to their apartment. As they wait in the dark for the killer, a man appears from the shadows and begins to struggle with Jeff. With police sirens wailing in the distance, Nancy hits Jeff over the head by mistake and the killer escapes. When the police burst into the apartment, Bolling, one of the officers, hears a noise in the other room and begins to shoot. After Jeff reveals Bolling's assailant to be Hickory, the turtle, he proposes that the killer must be hiding in the apartment. When Jeff opens a closet door to prove his theory, the body of Lingle falls out, having been hit by one of Boling's stray bullets. Astonished by his own acumen, Jeff faints.