In 1939, at an Italian hotel overlooking the Mediterranean, singer Frederica Brown is "rescued" from an overzealous admirer by a distinguished-looking man, but soon realizes that the rescue was a set-up to meet her. The man introduces himself as "Mr. Imperium" who works in the family business, the government. After Imperium rents the room next to hers, Fredda, as Imperium calls her, learns that he is the internationally famous playboy Prince Alexis, heir to a European throne. At first Fredda resists Imperium's charms, but soon acquieces and dubs him "Al." They meet the next day and begin an idyllic romance of sailing, horsebackriding and biking, during which she introduces him to American customs and he teaches her Italian words such as "arrivederci," or "I'll be seeing you." At Imperium's villa, she learns that he is a widower who hopes that his six-year-old son will lead a more normal life than he. One night, Imperium receives a telegram informing him that his father is gravely ill. His prime minister, Bernand, promises to deliver a letter from Imperium to Fredda, but instead leads her to believe that there was no note and the prince is merely following his pattern with women. Twelve years later, when Imperium is in Paris, he sees a theater marquee advertising American actress Fredda Barlo's latest film. Meanwhile, in Hollywood, Fredda has become a star who is loved by her producer, Paul Hunter. While looking for the perfect actor to co-star in her latest film, Fredda arranges to go to Palm Springs and uncharacteristically drive there without her maid, Anna Pelan, who suspects that a recent mysterious call from Paris is the reason she went alone. Before Fredda leaves the studio, Paul wonders why she is having so much trouble finding a co-star for a film that she suggested to him, the story of an American girl who falls in love with a European king. Paul then proposes, but she begs him to ask again on Monday. On the drive to Palm Springs, Fredda stops at exactly 3:00 o'clock to call Mrs. Cabot, the owner of the ranch at which she always stays, asking her to rent the other room she had reserved to whomever wants it. A moment later, Imperium arrives at Mrs. Cabot's doorstep asking for a room. Mrs. Cabot's suspicious niece Gwen thinks that she recognizes him, but cannot remember where she has seen him before. When Fredda arrives, she and Imperium pretend not to know each other, but once settled in, she unlocks the door connecting their two rooms and they embrace. He then tells her that he was a prisoner in his palace during World War II and was sent into exile when a revolution broke out after the war. He also reveals that he was very upset when he learned what Bernand had said to Fredda. Within a few days, there will be a plebiscite in his country to determine whether he should be returned to power, but Imperium does not want to be king. That night, as Fredda and Imperium dine together, he considers what he will do in America and she casually tells him about the film, knowing that he has a beautiful voice and has always secretly wanted to be a singer. He thinks that he would be perfect as the movie's king, and the couple happily drive back to Beverly Hills after she telephones Paul to say that she has finally found the right actor and wants him to come to her house in the morning. The next day, as Fredda and Imperium enjoy breakfast, Bernand, who had been at Paul's house when Fredda called the night before, arrives. When Imperium says that he plans to abdicate, Bernand coldly informs him that his son is no longer at school in England but is now on the continent, preparing to take the throne if his father will not accept it. Bernand then leaves, but asks Imperium to meet him at the airport. Fearing that his son was forcibly taken from school, Imperium calls England, but learns that the boy left of his own free will, to do his duty. Imperium and Fredda now know what must be done, and at the airport, he says "arrivederci" as he boards the plane. Paul again proposes to Fredda, but she ignores him, saying through her tears, "he said 'arrivederci.'"