Old Tom Sharpleigh has been discharged because of his old age, and he is without money. Agnes, his granddaughter, is dependent upon him. She gets employment, much against the old man's will, in the home of Marian Danforth, who takes a dislike to the girl when she finds she knows her betrothed, Henry Edwards. Marian does not like the idea of her sweetheart knowing a servant girl. He presents Marian with a rich necklace. No one knows of it being in the house but Harry, Marian and Agnes. That night, Joe, the ourang-outang escapes from his cage, and entering Marian's room, steals the necklace and returns to his cage. His master, Bill Barstow, an associate of criminals, finds the necklace and decides to sell it. Agnes is accused of the crime and arrested. Her grandfather begs to see the room the crime was committed in, and Hoyt, the detective, humors the old man, whom he considers almost in his dotage. The old man sees the monkey's handprint, but Hoyt laughs at it, and shoves him from the house. The old man traces the footsteps to the zoo and follows Barstow to a low quarter of the city. Then he goes to headquarters and asks for police to search the house, but is laughed at again. But Henry, the donor of the necklace, who has visited headquarters to induce Agnes to tell where she hid it, is interested in her and in the old detective and accompanies him to the den where they are discovered by the crooks. Henry is overpowered and they are about to kill him, when the old detective, who has stolen away, arrives with a policeman. The crooks are mastered and the necklace regained. They return to headquarters and the girl is released. The chief tells Tom to come back to work, for brains are needed at headquarters. When the old fellow arrives the next day, he finds his desk ornamented with flowers and white ribbons. He goes back to work. It also looks, as the story ends, as if a romance is developing between Henry and Agnes.