In New York City's Bowery, Slip Mahoney goes with his friend Sach to collect a paycheck from detective Gordon Gregory Grogan, for whom Sach worked for a week. Grogan is on vacation, but Sach and Slip accept a case from Selena Webster, who claims her sister, Eleanor Williams, is missing. Slip and Sach then form a detective agency with their friends, and their first assignment is to visit Dr. Rolfe Carter, a psychic from whom Eleanor had been seeking advice after the disappearance of her husband Tom. After the boys are thrown out of Carter's office, they follow him to the Wentworth Arms apartments. Slip enters room 5C just as Carter is shot to death and is knocked out. When he revives, he sees Eleanor calling the police and finds a bundle of letters. Slip tells Eleanor that he was hired by her sister to find her, but she denies having a sister and conjectures that the killer worked for a man named Dr. Armand. In Louie's Sweet Shop, Slip pays an informer to identify Armand, and learns that he heads a syndicate that controls fortune-tellers. Under an assumed name, Slip pays Armand to read his fortune in order to get his fingerprints. Armand knows who he is, however, and holds him and Sach hostage. Selena, who is Armand's assistant, explains to Slip that Armand used fortune-telling to learn incriminating secrets about her and Eleanor, then tried to blackmail them. Eleanor, believing her husband Tom was dead, began a love affair, but when she received word that Tom was alive, she hired Carter to steal from Armand some love letters she wrote. Carter stole a bundle of letters, including Selena's, but Eleanor's were not among them, and before he could switch Selena's letters for Eleanor's, he was killed. Selena helps Slip and Sach escape, and they elude Armand's henchmen by posing as the "brain trust" of Professor Quizard on a radio game show. Later, at Louie's, Selena visits Slip, Sach and the boys, who ask her for an affidavit stating that Armand killed Carter. Armand enters with his henchmen, however, and waits for Eleanor. After she arrives, Slip's girl friend Alice, a waitress at Louie's, arrives with Tom and the police, and Armand is arrested. Slip then discovers that he and the boys made no money on the case, and tells Alice he cannot afford their dream honeymoon. After Alice smacks him, however, Slip quickly changes his mind.