After she learns that her stockbroker husband Gerald has squandered all of the family money on bad investments, Gwen Parker telephones Philip Seymour, a former lover, and begs his help. Philip meets Gwen at the Battery Park Aquarium, which is run by Bertrand B. Hemingway, an angry, defrauded client of Gerald. While Gwen, who has announced her intention to divorce Gerald, asks Philip for a loan, Gerald receives an anonymous telephone call about his wife's activities. Overcome with jealousy, Gerald storms into the aquarium and, in his fury, accosts Philip, who strikes back and knocks him unconscious. Philip then drags Gerald's body behind the aquarium and leaves him lying next to the penguin display tank. While Philip and Gwen are approached by Hemingway, schoolteacher Hildegarde Martha Withers leads a group of her students on a tour of the aquarium and inadvertently knocks over "Chicago Lew," a deaf-mute pickpocket. After Lew slips away from the police, Hildegarde discovers that her antique hatpin is missing and sends her students to hunt for it. To Hildegarde's relief, the hatpin is found, but as the group is about to leave, Hildegarde sees Gerald's lifeless body plop into the penguin tank. Soon after, Inspector Oscar Piper arrives with his men and begins to question suspects, which include Lew, who is discovered behind the tank, Barry Costello, a penguin-loving lawyer who has offered his services to Gwen, Hemingway, Gwen and Philip. Eventually Philip confesses to the murder, and although Hildegarde, who has become an active participant in the investigation, doubts his guilt, Oscar arrests him. However, when medical examiner Dr. Max Bloom reveals that Gerald was killed by a sharp point being driven through his right ear, Hildegarde, realizing that her hatpin was the murder weapon and that only she, Oscar, Bloom and the killer know this fact, advises Oscar to tell the press that Gerald was stabbed through the left ear. After Gwen is arrested as Philip's partner, Costello shows Oscar a note from Lew, which states that he can identify the killer. By the time Oscar and Hildegarde arrive at Lew's Tombs prison cell, however, Lew is dead, and Hildegarde and Oscar suspect foul play. At Philip and Gwen's trial, Costello puts Hildegarde on the stand and, during his accusatory questioning of her, reveals that he knows that Gerald was stabbed in the right ear. After Hildegarde proves that Costello was both Gwen's lover and Gerald's killer, Gwen and Philip are released from prison. Oscar then proposes to Hildegarde.