Dana Thornton has a niece, Eleanor. She is friendly with Cynthia Carew, the daughter of a Senator. After a ball one night, Cynthia finds her father dead in their machine. While the ball was in progress we see Cynthia and Lieutenant Lane, a naval officer. Lane asks her father for her hand and is refused and a quarrel ensues. It later transpires that before going to the hall, Carew wrote a letter to the Secretary of State saying he knows a certain American who is selling his country's secrets in South America. The letter goes on to say that the name of the spy cannot be divulged yet. And immediately following the writing of this letter the old man was found dead, showing that someone thought him dangerous. Cynthia is prostrated and Eleanor Thornton and her uncle come over, the man to take charge of Carew's affairs and the girl to comfort her friend. Eleanor has a mysterious maid who is acting the part of the spy. Douglas Hunter, a secret service agent, is sent to help Detective Brett on the Carew case. The lieutenant who quarreled with Carew is under suspicion and Cynthia feels that he must be guilty and refuses to see him. Carew's nephew, Phil, is also suspected, because he had been seen quarreling with the Senator. Then the Stranger appears. He is Harry Thornton, brother of Dana, and his exact counterpart. Harry visits the Secretary of State, learns of Carew's death and has an attack of vertigo. When the Secretary goes to get him a drink he rushes down the hall. Later Dana visits the Secretary, steals the important papers and the stranger, who rushed away, is suspected. Eleanor sees Barry shrinking in the grounds and aiming a gun at the window. This is during a visit of the whole party as the guests of Dana. Phil is seized with illness. It transpires that Cynthia and Eleanor have kimonas just alike. One of them, or so it seems, visits Phil and gives him medicine and he, in delirium, tells where the missing plans are. She goes downstairs and finds them and tears her kimona. Hunter, the detective, sees her as she goes upstairs. The girl's kimonas are examined next day, but no trace of the tear can be found. Consequently it was not one of the girls that was downstairs the night before. Brett and Hunter search the maid's room. There is found the torn kimona and they believe that she was the mysterious girl of the night before. The doctor discovers that Phil is being systematically poisoned. Lieutenant Lane, Cynthia's sweetheart, is arrested for the murder of Carew. Dana sees Eleanor's maid listening at the door and finds her stealing papers in the library. That night Harry Thornton comes into the Thornton house and secrets himself in an untenanted room. No one sees him or knows he is there but an old colored servant. Cynthia tells Eleanor of a dream wherein she sees a door with panels in the shape of a cross. She goes to the door and tries to get out but cannot. She awakens screaming in terror. She also tells Cynthia that when she has this dream something terrible always follows. The mysterious maid lies down for a sleep and an arm reaches through the wall and extinguishes her light, leaving the gas on. She dies. Cynthia, coming down, passes the door and seeing the panels in the shape of a cross, screams and several men, to quiet her, force the door open and the body of the maid is found. In the girl's closet is the torn kimona and the missing plans which she stole on the night Hunter saw her. The maid was a secret service spy. All gather in the drawing room. Phil rushes in and accuses Eleanor of the murder of Carew and of poisoning him. He tells how he saw her go down to the machine on the night of the murder and return to the house frightened. She did this because she saw Dana, her uncle. commit the crime. A letter to the Secretary of State is found in a book. It was written by Carew before his death and forgotten. This is sent to the conference in the drawing room, where it is opened and the name of Dana Thornton is found therein. He is the spy. He jumps into a car and is followed by Detectives Brett and Hunter in another car. Dana jumps out of the car and hides, while the car then goes over a cliff. He rushes home and comes face to face with Barry. Barry gives him a gun and tells him it is the only honorable way. Dana shoots himself and falls dead. The crowd rushes in and Barry says: "He has paid the penalty." Eleanor recognizes Barry as her long-lost father and rushes to his arms. Barry tells about the time, years before, when he and Dana were in Panama. How he followed him to a saloon where friends of Dana's hit him on the head, and then he is sent to jail. He is offered his freedom by soldiers of the small republic for certain secret codes in his possession, but he refuses. He escapes by changing clothes with the guard of the jail. Then he goes to his own country to be revenged on his brother. Hunter and Brett arrive and they learn that Dana did not go over the cliff in the machine, but that he lies dead in the drawing room. Cynthia and Lieutenant Lane are brought together and Hunter and Eleanor have fallen in love, and the long-lost father gives his blessing to their union.