Sally and Dan Brogan live with their father in a poor quarter of the city. Mr. Brogan is a man of quick temper and the children are often ill-treated when he is in these ugly moods. Dan sees a tattoo mark on the shoemaker's arm, and wants to have his arm tattooed also. To gratify his wish, he breaks open his father's tin savings bank and steals the money from it. Dan shows his sister, Sally, the tattooed design on his arm, with their two names, in the shape of a cross. When his father discovers the loss of the money, and finds Dan has stolen it, he thrashes him. Dan runs away from home and falls in with two crooks, who take him in hand and make him as crooked as themselves. Fifteen years pass and Dan, now an accomplished thief, is still with his two companions. They plan to steal a famous diamond which is on exhibition in a jeweler's shop window. This is successfully accomplished and they hide it in the cellar of the house in which they live. The task of recovering the diamond and capturing the thieves is placed in the hands of Miss Sally Brogan, now a member of the Secret Service Police. She skillfully discovers the identity of the robber, and tracks him down, but is herself tracked by Dan and made prisoner by him. By chance she sees the tattoo mark on his arm and recognizes him as her own brother. He also recognizes her by a crescent-shaped locket he had once given her and they plan to escape that night. His two former pals become suspicious and watch him closely. Dan helps his sister to escape and is shot and mortally wounded. The two crooks determine to make off with the great diamond and go into the cellar after it. Dan, although suffering from his wound, crawls to the trap door leading to the cellar, closes it and throws his body across it, entrapping the two robbers in the cellar beneath. Meanwhile, Dan's sister has obtained help, and the precious stone is recovered.
—Moving Picture World synopsis