Olivia DeHavilland is a princess, staying in the royal suite as a hotel. Charles Coburn is her uncle. He is the ambassador of a European country to the United States. She has many suitors but is not interested in them. He suggests that she go to a western ranch near San Fransisco to get away from him and the Washington scene. On the plane she is unable to sleep and takes three sleeping pills (never having taken them before). One of the pilots is Bob Cummings. He asks her for a fourth pill, and he insists that she take another. Then she passes out. The plane has to return to NY because of fog. When they land, they have to revive her and Cummings walks her to the vehicle. He cannot learn where she lives. At a cafe where they stop, the owner puts a couple of sleeping pills in her coffee, and out she goes again. He takes her to his apartment, but calls Jack Carson, his co-pilot, to have his wife (Jane Wyman) undress her.
When she wakes up, she goes home, but tells Coburn that she slept in the ladies' lounge at the airport. Then she begins trying to reconstruct the events of the past day. Then she heads out to try to find Cummings, her secret service agent in close company. She recognizes him, and they enjoy a cherry Coke together. When he begins to question her about her live, she is evasive. He volunteers to show her the town and they get Carson and Wyman to go with them, but first she goes with Wyman to a volunteer organization. DeHavilland is getting a taste of the common life, and is loving it. When the volunteer coordinator asks her what she is able to do, she finds that she cannot do anything useful, but asks to be able to do something useful for the afternoon. Her job is to be a "dummy" for a first-aid competition. At dinner, she makes up a fictitious life story. They walk home he asks her to marry him. She tells him she wants to, but never says she will.
The secret service man and Coburn get a report from a private detective agency on Cummings. Coburn calls the King and tells him about the romance - positively. De Havilland calls Cummings to tell him she cannot marry him, but he has to catch his plane and cuts her off without understanding. The Coburn talks to him and tells her that it might be politically expedient, given the times, for her to marry an American boy - an ordinary boy. The next time he calls, she accepts the proposal. When Wyman meets Carson and Cummings at the airport, she tells them that her apartment was registered in the name of a middle-aged man. She takes Cummings to the St. Charles Hotel to meet her uncle. And then the King calls to talk to him. He learns he will be a Prince Consort.
On the train to Washington to visit the President, the Ambassador schools him on protocol with royalty. When he finds out all the requirements, including giving up his citizenship, he tells De Havilland he will only marry her as a common couple. Then he walks out, but he has no place to sleep in war-time Washington, so he comes back later. She cries on her bed, and then gets an idea. She writes a note and gives it to FDR's dog (who evidently plays himself in the movie). The dog takes the note to the President, who summons a Supreme Court justice and there is a surreptitious wedding ceremony in the President's office. As they leave, he unknowingly bumps into the President and, thinking he is a guard, tips him a dollar.
REALLY enjoyable movie.