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Philip Waring, head of the Cipher Bureau, is trying to break up a Washington spy ring which is transmitting radio messages in code. Philip discovers some of their code books in an unsuccessful raid, but ringleaders Simon Herrick and Albert Grood elude him. The next day, Philip arranges to have his Naval officer brother Paul sent on a mission to pinpoint the new transmitting station. Eager for a chance to work with the bureau, Paul goes to the Brooklyn Navy yard to analyze the signals received. On the train back to Washington, Paul is seated next to the beautiful Therese Brahm, who is secretly working for the spies and ingratiates herself to him. Meanwhile, Philip realizes that the signals Paul detected were decoys, but he and his men trace one to a remote cabin in Virginia. After a skirmish, they capture one radio operator and take his code books back to Washington. During a lengthy decoding process, Philip learns that the spies want naval plans for an improved version of the old French 75mm gun. Helen Lane, Philip's secretary, is elated when he invites her to a swank restaurant that night to celebrate, but soon discovers that Philip knows it is frequented by the spies, and wants to intimidate them. Philip sees Paul and Therese at the restaurant and later asks his superior, Commander Nash, to assign Paul to a special duty, but not tell him the real assignment, infiltrating the spy ring. While on the train to his new duty, Philip is drugged, and the next day Nash announces that Paul was taken off the train intoxicated, and the secret papers in his care are missing. At Paul's court martial, Philip cannot reveal important information for national security reasons, and Paul is dishonorably discharged. Because Paul blames his brother and refuses to see him, Philip goes to Therese, who does not know where Paul is, and is genuinely worried. Philip then tells her that the stolen papers were not really valuable, hoping she will pass the information on. Later, when Paul goes to see Therese at the restaurant, she refuses his proposal because she fears for his safety, and he leaves. When Philip comes to the restaurant, Grood and Herrick warn her not to cross them, because they suspect her real affection for Paul. Therese summons Philip to the office, where they hold him at gunpoint, and pretends to be against him. When some military police arrive, interrupting Grood and Herrick's plans, and say that a woman had phoned in a tip, Philip knows that Therese really cares for Paul and has helped him. When she returns home, Grood and Herrick say they are all going away and she asks to write a goodbye letter to Paul. Paul soon shows up at Philip's office and shows him the letter, saying that Therese has disappeared. Philip realizes that it is written in code and deciphers her message, which reveals that a radio program at 8:00 p.m. will transmit a secret message in a musical piece played by Herrick. Philip's men record the music and decode the notes to reveal the message that the spies will escape that night on a sea plane meeting their ship. Meanwhile, Paul, who has been following two of the spies for Philip, is captured and taken on board the ship. Therese is also on board and the two are rescued by Philip and his men, who arrest all the spies. With their work done, Helen and Philip plan a trip to Niagara Falls, and Paul and Therese decide to make it a double honeymoon.