A honeymooning couple are asked to spy on the Nazis in pre-war Europe.

In spring 1939, American-born Oxford professor Richard Myles and his American bride Frances begin their honeymoon at an English roadside inn, registering under the names Mr. and Mrs. Edward Smith. Their first night together is interrupted by the unexpected arrival of Peter Galt, a former classmate of Richard's, now employed by the Foreign Office. Aware that the newlyweds are headed for southern Germany, Peter asks Richard and Frances to track down a scientist "friend," who has information about a magnetic mine the Germans have developed. Richard and Frances eagerly agree to help, feeling that as American tourists, they are "above suspicion." Upon arriving in Paris, Frances buys a hat decorated with a rose, which is to serve as a signal for their contacts. In a Parisian restaurant, Frances and Richard meet up with their first contact, who silently instructs them to go to a café in Montmartre. There, a guide book to southern Germany is slipped into Richard's coat pocket. Frances and Richard notice a series of pinpricks on a map in the book, and Frances discovers that, when connected, the dots become the opening notes to the song "My Love Is Like a Red, Red Rose." The guide book also indicates that the couple is to go to Austria, to a certain bookstore in German-occupied Salzburg. At the bookstore, the newlyweds mention the song title, and under the noses of some Gestapo officers, the owner, A. Werner, signals them to head for a Salzburg museum. There, a tour guide named Hassert Seidel suggests that they check into a guest house run by Frau Kleist. After a few uneventful days at the guest house, Frau Kleist gives Frances and Richard a biography of Franz Liszt, aware that the couple is planning to attend a concert of Liszt's music that night. From markings in the book, Richard and Frances deduce that they are to go to a village called Pertisau, near Innsbruck, and inquire about a man who collects chess pieces. Before leaving for the concert, Richard meets fellow boarder Thornley, a recent Oxford graduate. Although Thornley, who has been practicing a noisy passage from Liszt's first piano concerto, advises Richard not to attend the concert, the couple goes anyway. At the concert, as the orchestra plays the same passage that Thornley had been practicing, a Nazi colonel is shot and killed. Nazi officials insist on questioning the entire audience, but Richard and Frances are rescued by Count Sig von Aschenhausen, a former Oxford schoolmate of Richard's. At Richard's behest, the count also intervenes on Thornley's behalf and invites all three visitors to his mother's nearby estate. There, Sig comments that the assassin must have been familiar with the Liszt concerto and asks Thornley to play the passage for him. Thornley claims not to know the piece, but later Thornley admits to Richard that he killed the Nazi as revenge for torturing and killing his Austrian girl friend. The next day, in Pertisau, the owner of a collectors' shop directs the newlyweds to the home of Dr. Mespelbrunn. While waiting to see the doctor, Richard and Frances notice the sheet music for "My Love Is Like a Red, Red Rose" on his piano. Just then, Sig appears, explaining that he has come to see Mespelbrunn on important business. When Sig does not respond to a signal that Richard gives him, the couple becomes suspicious. They then hear thumping noises upstairs and discover that Sig is holding Mespelbrunn prisoner. Before Frances and Richard can free the doctor, Sig and his Nazi cohorts interrupt them. The Americans flee to the surrounding woods, where they are surprised by the timely arrival of Hassert. With Hassert's help, Frances and Richard free Mespelbrunn, and all four head for Innsbruck. Outside Innsbruck, a now-disguised Mespelbrunn gives Richard a paper with the mine information on it, states the names of several trustworthy men, then goes his way. Hassert drives Richard and Frances, who are disguised in Tyrolean clothes, to Innsbruck, instructing them to obtain passports from a couple named Schultz. At the Schultzes, Richard and Frances dress as elderly people and have their photographs taken for their forged identification. After sending the newlyweds on their separate ways, the Schultzes are arrested by the Gestapo, who discover the negatives from Richard and Frances' passport photos. As she is about to reunite with Richard, Frances is spotted by Thornley, who also happens to be in Innsbruck. Frances is then picked up by the Gestapo, but manages to tell Thornley where to meet Richard. Thornley alerts Richard to his wife's plight, and Richard seeks Hassert's help. Waiting until most of the Gestapo have left headquarters for the night, Richard, Thornley and Hassert sneak in and, with guns blasting, take Frances' interrogators by surprise. The men steal some Nazis' uniforms and flee with Frances, who has been beaten, but as they are escaping, Thornley is killed. In a Nazi car, Hassert and Richard hide Frances, then drive to the Italian border. After fooling the Nazi border guards, Hassert and the newlyweds finally find freedom.