Cariño mío (1961)

Miguel and Verónica meet on a plane and lie to each other about their true selves: she tells him she's a secretary but she's the billionaire niece of a duchess; and he, a Teutonic king, tells her he's a trumpet player. Eventually they fall in love without knowing about each other's true identity; meanwhile, Miguel's ministers are urging him to choose a bride of royal blood and get married as soon as possible, and a revolutionary group--including Verónica's brother--is planning a terrorist attack against the king.