At Grainbelt University, a Midwestern university, freshman Dobie Gillis (Bobby Van) and his pal Charlie Trask (Bob Fosse) court cute coeds Pansy Hammer (Debbie Reynolds) and Lorna Ellingboe (Barbara Ruick). Pansy's wealthy father George (Hanley Stafford) can't stand Dobie and does everything in his power to keep them apart. Along the way, Dobie and Pansy manage to blow up the chemistry lab, while Dobie's officious English professor Pomfritt (Hans Conried) is misled to believe that the feckless Gillis is a literary genius.
Rodolphe Rassendyll, un touriste anglais, arrive à Strelsau, capitale d'un pays imaginaire d'Europe Centrale, la Ruritanie. Il y rencontre un lointain cousin dont il est le parfait sosie, le prince héritier Rodolphe V. Celui-ci doit être couronné Roi le lendemain, mais à l'issue de la soirée que les deux parents éloignés passent ensemble, le futur souverain ne peut être ranimé : il a bu un vin drogué par une complice de son demi-frère, Michael de Strelsau, lequel escompte se proclamer Régent du Royaume, en l'absence à la Cérémonie du Couronnement de Rodolphe V, puis faire assassiner ce dernier pour accéder ainsi au Trône. Mais deux fidèles du prince légitime, le Colonel Zapt et Fritz von Tarlenheim, font accepter par Rassendyll qu'il joue le rôle de souverain au couronnement, grâce à cette providentielle ressemblance. Les choses se compliquent lorsque Rodolphe V, toujours endormi, est enlevé le même jour par le mercenaire Rupert de Hentzau ; en outre, Rassendyll tombe amoureux de la Princesse Flavia, promise en mariage au prince héritier.
In 1934, impoverished painter Eben Adams (Joseph Cotten) meets a fey little girl named Jennie Appleton (Jennifer Jones) in Central Park, New York. She is wearing old-fashioned clothing. He makes a sketch of her from memory which involves him with art dealer Miss Spinney (Ethel Barrymore), who sees potential in him. This inspires him to paint a portrait of Jennie.
In the latter months of 1938, Laura Jesson (Celia Johnson), a respectable middle-class British woman in an affectionate but rather dull marriage, tells her story while sitting at home with her husband, imagining that she is confessing her affair to him.
Social outcasts Mary Marshall (Ginger Rogers) and Sgt. Zachary Morgan (Joseph Cotten) meet while seated across from each other on a train bound for Pinehill. Zach, a victim of shell shock and, therefore, a prisoner of his own mind, has just been granted a ten-day leave from a military hospital to try to readjust to daily life. Mary, convicted for Involuntary Manslaughter, has just been given a special eight-day furlough from prison to spend the Christmas holiday with her aunt and uncle in Pinehill. Each harbors his own secret. Mary lies to Zach that she is a traveling saleslady on her way to spend the holidays with her family, while Zach tells Mary that he is going to visit his sister in Pinehill. After the train pulls into the station, the two exchange names. Mary then goes to the Marshall home, where she is reunited with her Uncle Henry (Tom Tully), Aunt Sarah (Spring Byington) and cousin Barbara (Shirley Temple).
Irene Vail (Jean Arthur) decides to divorce her husband, the rich ship owner Bruce Vail (Colin Clive), after he falsely accuses her of having an affair. Bitterly jealous and possessive of Irene, Vail learns that he can prevent the divorce from being finalized if he can provide evidence that she has been involved with another man within six months of filing for divorce. Vail pays his driver, Michael (Ivan Lebedeff), to go to Irene's hotel room in Paris and pretend to be her lover, with the intention of having a private detective catch them in a compromising position. However, an unknown man overhears Irene's startled cry upon finding Michael in her room. A struggle ensues when the man defends Irene against Michael's unwanted advances, and ends with Michael on the floor, unconscious. When Vail and the detective burst into the room, the man threatens them with a gun, demands Irene's jewelry, and takes Irene hostage.
English gentleman Rudolf Rassendyll (Ronald Colman) takes a fishing vacation in a small middle European country (unnamed in the film; Ruritania in the novel). While there, he is puzzled by the odd reactions of the natives to him. Rassendyll discovers why when he meets Colonel Zapt (C. Aubrey Smith) and Captain Fritz von Tarlenheim (David Niven). Zapt introduces him to the soon-to-be-crowned king, Rudolf V (Colman again), who turns out to be not only his distant relative, but also in facial features close enough to be his double. Rudolf is astounded and takes a great liking to the Englishman.
The story takes place in the Russian Empire during the last years of the reign of Czar Nicholas II and the Czarina Alexandra. Reform-minded Prince Paul has long been concerned about the plight of the common people and knows a revolution is brewing. Prince Alexei, heir to the throne, is loved by the people but has hemophilia, and a slight fall turns out to be life-threatening. When royal physician Dr. Remezov is powerless to stop the boy's bleeding, Princess Natasha, Alexandra's lady-in-waiting and Paul's fiancee, recommends Rasputin as a healer. He convinces the frantic Empress that he has been sent by God to cure the child. Left alone with Alexei, he hypnotizes the boy and relieves his agony but also gradually makes Alexei a slave to his will.