Boston Blackie, a gentleman crook, and his accomplice, Mary, plan to rob the Wilmerding mansion while Mr. Wilmerding is out of town. Mary is hired as a nurse to Martin Wilmerding, Jr., and after Mrs. Wilmerding has gone to a ball, she admits Blackie, who starts to open the safe. Just then little Martin enters, and he and Blackie play and become fast friends. Mrs. Wilmerding returns with her lover, Donald Lavalle, and when Blackie overhears their plans to elope together with her jewels, he tricks Donald into giving him the jewels by posing as the jealous husband. Through several telegrams, he effects the reconciliation of his little pal's parents but cannot decide whether or not to return the jewels.