The Prisoner of Zenda (1937)

It's the late nineteenth century. British Major Rudolf Rassendyll is in central Europe on a fishing vacation, he having no intention of attending what is the reason others are in the country, namely the coronation of Rudolf V to the throne the following day.

In a chance encounter with Rudolf V and two of his longtime trusted advisers, Colonel Zapt and Fritz von Tarlenheim, in the countryside, Rassendyll discovers that he and the future King are not only distant cousins, hence both having the long held family male given name of Rudolf, but that they could be twins. Spending the evening with the three men, Rassendyll further discovers the future king is an irresponsible heavy drinker, something that his older half brother, Michael, wants to use to ascend to the throne himself. Michael, using his own operatives, has managed to drug Rudolf, heavily enough to miss the coronation ceremony, at which time Michael plans to address the people to name him king instead. With risk to all three of their lives if discovered, Zapt and von Tarlenheim are able to convince Rassendyll to masquerade as Rudolf at the coronation. Another risk is Michael using other means to dispose of who he believes is his brother. Michael, however, has other issues with which he has to address. Antoinette de Mauban, Michael's girlfriend, does not want him to be king as such would mean marriage to who has been the king's long ago named betrothed, namely Rudolf and Michael's cousin, Princess Flavia. In turn, another of Michael's operatives, Rupert of Hentzau, is in love with Antoinette himself, and thus has his own self-directed motivations different than Michael's. Conversely, other complications arise as Rassendyll, in posing as his cousin, falls in love with Princess Flavia himself. Flavia, who has had a long relationship of animosity with Rudolf who she does not love let alone really like, has fallen in love with this new version of Rudolf. She may be torn if she ever discovers the truth of the deception as she has long been bound a sense of duty, which means marrying the king, whoever he may be.