Ralph Cornell, a Secret Service agent, trails a counterfeiting gang to their headquarters in the Old Coddington Mansion, a country residence run as a boarding house by Mrs. Quail and her daughter Betty. The two women are unaware that their boarder, Aaron Prine, is the counterfeiter's chief and that an underground den exists with a secret passage leading to it through the walls of the house. Cornell becomes a boarder, and he and Betty fall in love. Prine suspects Cornell, and there ensues a battle of wits, followed by a fist fight between them in which Cornell is beaten. After many adventures, Cornell obtains his evidence but is trapped with Betty in the cellar. His men arrive in time to rescue their leader and his sweetheart, and the counterfeiters are arrested.
