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In New York City, soon after a typed note reading "Who killed Cock Robin?" and signed by "The Bishop" is found in the mailbox of prominent scientist Professor Bertrand Dillard, the body of his young assistant Robin, nicknamed "Cock Robin" by his friends, is discovered on the archery range on the grounds of Dillard's home. Detective Philo Vance is asked by the district attorney to consult on the case. Gathered at the scene of the crime are John E. Sprigg, the victim's best friend, who vows to find the killer; Belle Dillard, the professor's niece and ward; and Sigurd "Eric" Arnesson, Belle's fiancé and Dillard's assistant. After Vance interviews Dillard's neighbors, the elderly Miss Drukker and her hunch-backed brother Adolph, whom Vance is sure witnessed the crime, Miss Drukker dies of heart failure. Adolph is murdered one night while sitting on a wall in the park, reminiscent of the nursery rhyme "Humpty Dumpty," which is included in another note from "The Bishop." As typed notes from "The Bishop" continue to appear, each with a reference to a nursery rhyme, suspicion is soon cast on Arnesson, and Vance deduces that "The Bishop" is not a reference to a cleric but a chess piece. One night, when Belle hears a loud typing noise in an upstairs room of her house, she is confined by the unseen typist. Later, while Vance and the others are looking for Belle, Arnessen is brought before Vance and implies that he is guilty. Moments later, Dillard is uncovered as "The Bishop" when he drops poisin from his ring into a glass intended for Vance, who has discovered that Dillard secretly hated Arnessen for robbing him of his scientific fame and taking over the affections of Belle. When his guilt is revealed, Dillard drinks from what he thinks is the poisoned glass. After Dillard dies, Arnessen and the others think that Dillard drank the poison until Vance reveals that he had switched glasses and that the professor had actually died of shock. Belle, who was not harmed by her uncle, is reunited with Arnessen, who had pretended to act guilty after Vance slipped him a note to "play along."