John Holden discovers a burglar in his house and shoots at the escaping thief. Warding, a detective, and the officer on the beat hear the shot and hurry to the scene. The detective finds a large diamond set on the library rug and concludes it has been lost from the thief's ring. Thinking it probable the crook will try to recover the diamond, Warding secures a paste duplicate which he places on the rug and awaits developments. The accomplice, Nancy, is sent to the house in the guise of a book agent. Admitted to the library, she pretends to faint and while Mrs. Holden is getting a glass of water the girl searches for the stone and finds it on the rug. When Nancy departs Warding, who has been watching the affair, trails her to the thieves' hiding place and obtaining the assistance of an officer, he succeeds in arresting the criminals.