Cariño mío(1961)

Miguel and Verónica meet in a plane and lie to each other about their true self: she tells him she's a secretary while she actually is the billionaire niece of a duchess, and he tells her he's a trumpet player while he is a Teutonic king. Eventually they fall in love, without knowing about each other's true identity, all of this while Miguel is being urged by his ministers to choose a wife of royal blood and get married as soon as possible, and a revolutionary group, one of which members is Verónica's brother, plans a terrorist attack against the king.