A policeman doubles as a gunman to get in with the mob.

Crooked detective "Big Ben" Benson, secretly allied with a gang of thieves, arrests young Tim Brady for murder during a robbery. When Tim is sentenced to die, "Diamond Joe" Jennings, the gangster who framed him, promises Tim's sister, Nora Brady, that he will use his influence with the governor to free her brother. However, Nora takes refuge with Diamond Joe and his companion, Kitty, and learns that Joe wants Tim to die, so he can cover his own crime. Nora informs the police, who assign Dan Hogan, brother of murder victim Pat Hogan, to aid her. Under the alias of "Scar" Sherman, Dan is introduced to Diamond Joe, and with the aid of a dictograph he learns of Tim's innocence. At a dance, the gangsters expose Dan as an undercover officer, but the police, called by Kitty, arrive in time to save him. Big Ben Benson shoots Diamond Joe to protect himself, and Dan arrests him. Tim's execution is stayed, and Dan and Nora find happiness together.