Based on a novel by Herbert Yardley, who was head of U. S. Secret Service in World War I, a code expert helps capture a German spy ring despite romantic flirtation.
In April 1917 German submarines blow up ships in a danger zone. Rendezvous position for ships is sent in code. At a Washington party Lt. Bill Gordon (William Powell) tells the Russian ambassador (Henry Stephenson) he is no longer a newspaperman, and he flirts with Joel Carter (Rosalind Russell). Captain Nieterstein (Cesar Romero) reads a secret message and calls Dr. Jackson in San Diego to let the munitions ship through. Jackson drives to Mexico, where the message is radioed to the Germans. Joel marches for women's vote and fights with a policeman, Bill helping her. Joel kisses Bill at the train, but he has another day. He wrote a book on codes by another name, and she gets her uncle John Carter (Samuel S. Hinds) to transfer him to the War Dept. Major Brennan (Lionel Atwill) tells Gordon to decode an enemy message. In the decoding room Gordon works late. Joel puts sleeping pills in his coffee, and he takes a drug to keep awake. With his leadership they crack the code and realize their own code was stolen.
Brennan calls Olivia Karloff (Binnie Barnes) to come over and tries to find a lost briefcase. He has a driver take a briefcase to Olivia and catches her steaming open an envelope. She kills Brennan, telling Nieterstein and De Segroff. Olivia mourns at Brennan's funeral. Gordon has Professor Martin (Charlie Grapewin) examine a circular sent to Olivia. Gordon finds Joel in his office and agrees to dine with her. Olivia asks Gordon to explain, and he asks her to take her clothes off, with Joel's assistance. Gordon questions Olivia about Brennan and has her followed. Olivia goes to a Russian church and leaves a message for Nieterstein.
Olivia calls Gordon, and he dines with her. She asks where he got the circular. Joel arrives with Nieterstein, who is called away. Martin calls Gordon and is shot; Nieterstein gets the circular. Olivia drops Nieterstein's medal in Gordon's water, and he sees it has a re-agent. With Olivia Gordon goes to the Ambassador about the re-agent. Gordon finds a radio and a code book. Nieterstein jumps out the window. Gordon tells Carter he can send the rendezvous position now. Gordon takes Olivia to her new hotel. Joel follows and insists on seeing Olivia; but she is taken to another room. Gordon grabs the note to Olivia; but the steward and elevator operator pull guns on Gordon. De Segroff asks Gordon for the new code, showing him captured Joel. Gordon gives a false position. Olivia calls Dr. Jackson in San Diego. Gordon kisses Joel, and a gun shooting at them misses. Carter and the police come in. Gordon tells them to arrest Jackson. Joel blames Gordon before realizing they were spies. In the final scene Gordon kisses Joel at the train but is again transferred to the War Dept.
This spy thriller combines romantic comedy so that the hero can help win the war and the girl at the same time, enabling an audience to see complicated espionage solved and courtship leading to marriage in 94 minutes.