At the local aquarium, Gwen Parker arranges to see her former beau, Philip Seymour, asking him for money so she can leave her husband, Gerald Parker. Her husband, tricked by an anonymous tip about the rendezvous, confronts the pair and is knocked unconscious by Seymour. With no witnesses to the crime, Seymour stuffs the unconscious man behind a display and hopes the affair will stay hidden. The stage is set for a tense, tangled web of motives and alibis that will pull the aquarium into a deeper mystery.
Schoolteacher Hildegarde Withers leads her class on a field trip to the same aquarium and quickly becomes an unlikely investigator in her own right. She saves the day when she foils a pickpocket, Chicago Lew, by tripping him with her umbrella, though the thief manages to slip away. The day’s mishaps continue as she discovers her hatpin is missing, only for a student to return it to her. Then a terrible sight catches her eye: Parker’s body plummeting into a pool home to penguins.
Inspector Oscar Piper arrives on the scene and begins weighing suspects. The list includes the widow and divorced Parker’s lover, Philip Seymour; aquarium head Bertrand Hemingway, who had financial dealings with the deceased; and Chicago Lew, found near the crime scene. By the time bystander-turned-lawyer Barry Costello helps Gwen Parker when she faints, he has also become Seymour’s attorney, placing him at the center of the unfolding drama.
Miss Withers proves invaluable to Piper, taking careful notes, typing up interviews, and highlighting clues that his quick mind might have missed. Her involvement grows steadily, but it becomes unsettling when investigators determine that Hildegarde’s hatpin was driven through the victim’s right ear, a gruesome detail that forces her to confront the possibility that she herself is under suspicion.
Seymour’s confession is a strategic move to shield Gwen Parker, yet Miss Withers remains unconvinced. She convinces Piper to inform the press that the murder involved a left-ear thrust, attempting to shape the narrative in the slightest way that might reveal the truth.
A new lead arrives when Costello relays a message from [Chicago Lew], suggesting he knows the killer’s identity. When Piper and Withers visit Lew at the jail, they find him dead from hanging. Costello then demonstrates how Seymour could have escaped his cell using a duplicate key, strangled Lew, and hanged him with wire—without ever entering Lew’s cell—revealing a possible chain of coercion and misdirection.
In the courtroom drama that follows, Costello challenges Miss Withers, but slips up by showing he knew the murder was carried out with a right-ear wound. The motive becomes clear: Costello himself is Gwen’s lover, and he killed Lew to keep his own grip on Gwen’s affections.
When Gwen Parker is finally released, Seymour confronts her with a brutal slap, drawing sardonic amusement from Piper and a wary sympathy from Withers. In a surprising turn, Piper asks Hildegarde to marry him, and she accepts, closing the chapter on this particular case while hinting at the couple’s ongoing partnership in a future installment—Murder on the Blackboard.
