The Penguin Pool Murder

Gwen Parker meets her former boyfriend Philip Seymour at the local aquarium and pleads for money to leave her husband. Gerald Parker receives an anonymous telephone tip about the rendezvous and confronts the pair. Seymour knocks Parker out and hides the unconscious man in the room behind an exhibit.

Hildegarde Withers leads her class on a field trip to the aquarium and helps stop a pickpocket by tripping him with her umbrella. The thief slips away, Withers loses her hatpin (found by a student), and Parker's corpse is soon seen falling into the penguin pool.

Inspector Oscar Piper arrives and begins to identify suspects, including Gwen Parker and Seymour; Bertrand Hemingway, the aquarium head with financial ties to Gerald Parker; the widow; Chicago Lew, found hiding near the scene; and Barry Costello, a bystander who becomes Seymour's lawyer.

Miss Withers assists Piper by taking notes and typing up interviews, earning his appreciation. She later becomes a suspect when investigators determine the hatpin punctured the victim's brain, implying a murder through the right ear.

Seymour confesses in an attempt to protect Mrs. Parker, but Miss Withers doubts him. She persuades Piper to notify the press that the murder was committed with a thrust through the left ear, shifting public focus away from Seymour.

Barry Costello passes along a message from Chicago Lew claiming to know the killer's identity. Piper and Withers go to Lew's jail and find him dead from hanging. Costello then explains a plausible way Seymour could have escaped Lew's nearby cell using a duplicate key, allowing a strangulation by wire without entering Lew's cell.

At the murder trial, Costello questions Miss Withers and reveals that Gerald Parker was killed via a wound to the right ear. He also implies that he tipped Seymour and killed Seymour himself to win Gwen’s affections.

Gwen Parker is released, and the waiting Seymour slaps her in the face, much to the amusement of Piper and Miss Withers. In the moment, Piper asks Miss Withers to marry him, and she accepts.

Costello's role grows as he moves from bystander to Seymour's lawyer and remains deeply involved in the case's outcome, signaling his increasing influence in the proceedings.

In a final twist, it is revealed that Costello killed Seymour to win Gwen Parker, having manipulated the tips and events to serve his own romantic interest. The motive centers on possession and jealousy, rather than a straightforward killer-for-hire plot.