The Hennion gang of crooks having killed Joe Kridel, a police inspector, and fearing discovery, accept the aid of Simmons, who wears a gray mask that covers wounds caused by an explosion. Jim Garth, another inspector, is placed on the trail of the gang of which he becomes a member. Nora, the daughter of Garth's inspector, is beloved by Garth. She appears to be one of the gang. Although Jim loves her deeply, she is cold to him. She has sworn to avenge the murder of Kridel, and believes the murderer to be one of the gang. The Hennions attempt to steal a valuable explosive and penetrate to a steel room in which the chemical is placed. Nora learns the name of Kridel's murderer. When the gang has gathered in the steel room, Garth reveals himself and calls in the aid of the police. The gang is arrested; the murderer secured, and out of gratitude, Nora accepts Jim Garth as her lover.
—Moving Picture World synopsis