Mrs. King, wife of a millionaire, plans to give a masquerade ball. An epidemic of jewel robberies causes her husband to hire Detective Burke and his assistant, Mary Macey, to attend the affair. Burke informs King that his costume for the evening will be that of a burglar, while Mary will be clad as a cowgirl. King jots this down in a notebook. The book later drops out of his pocket and is found by Diamond Al, a notorious crook. The man resolves to attend the affair disguised similarly to Burke. Burke reads of a $20,000 jewel robbery. He believes it to have been Diamond Al's work. The detective describes the crook to Mary. According to Burke, Diamond Al has the nicotine-stained fingers of a cigarette fiend. On the night of the masquerade ball, Diamond Al, lurking on the veranda, sees King show the detectives the little safe in the wall which contains the family jewels. Later the crook enters the room. Burke catches him in the act of robbing the safe. A struggle ensues. Burke is stunned by the criminal. Diamond Al binds and gags his victim, after which he lays him on the veranda. The crook returns to the room just as Mary enters. Mistaking him for Burke, the girl requests that he dance with her. The mask on his face prevents Mary from discovering her partner's identity. Later, however, she sees the nicotine stains on his fingers. Burke revives and succeeds in freeing himself. Mary holds Diamond Al up with her gun. The man seizes the weapon from her hand just as Burke enters. The detective leaps upon the crook from behind. Despite Diamond Al's struggles, Burke makes him a prisoner.