Dr. Truman Harwell examines his longtime patient, Silas Leavenworth, a rich, old stockbroker, who wishes to give his fortune to charity to atone for having cheated his clients. Silas is accompanied by his lovely, young wife Gloria, with whom Truman is in love. After Truman assures Silas that he has many good years ahead of him, Silas invites him to a dinner party, which is also attended by Gloria, Silas' spinster sister Phoebe, Silas' niece and ward Elenore, and Henry Clavering, who is Silas' business partner and Elenore's fiancé. Silas announces at dinner that he is changing his will, and states that while he will provide the family with a small income, the bulk of his estate will go to charity. The family is greatly upset, especially Elenore, who threatens to stop him by any means. The next morning, Silas is found dead, after apparently committing suicide by leaving the gas on, but Inspector Holmes assigns Bob Gryce and O'Malley to investigate. Bob and O'Malley examine the house and question the residents, after which Bob goes over the books at Silas and Henry's brokerage firm and discovers that Henry has embezzled one million dollars from the Leavenworths. Phoebe presses charges and Henry is arrested. Meanwhile, Gloria hysterically questions Truman, who she believes is the killer, and Truman tries to convince her of his innocence by telling her that Elenore is responsible. Truman later forges a confession note at his office and instructs his trained monkey, "Jocko," on how to plant it in Elenore's room. He returns to the Leavenworth home and, after sedating Gloria, opens a vent in her room and sends Jocko to Elenore's room. The plan goes awry, however, when Jocko accidentally awakens Elenore after he drops the note. Frightened by her screams, Jocko runs to the kitchen, where Bob later finds him and gives him to Phoebe to look after. Later, when everyone is asleep, Truman stands outside Phoebe's window and whistles for Jocko, who removes a wooden darning egg from Phoebe's purse before rejoining Truman. Truman then has a syringe, which was hidden in the egg, analyzed and discovers that it contains arsenic. Phoebe, meanwhile, has consumed a raw egg in milk, just as Silas did every night. The next morning, Bob interrogates Phoebe, who admits that she injected Silas' egg with arsenic because she considered him a thief and a scoundrel. She also confesses that she intended to kill Henry next in order to keep him from marrying Elenore. Bob admits that he is in love with Elenore himself, after which Phoebe succumbs to the arsenic with which she has poisoned herself. Truman tells Bob that he had suspected Phoebe and had trained Jocko to replace Silas' nightly egg with an uncontaminated one, but it was too late. Truman admits he was responsible for the confession note, but asserts that it was to force Phoebe to admit her guilt. Truman agrees to go away with Gloria, and Bob and Elenore also arrive at a romantic understanding.