The Prisoner of Zenda is not only one of my favorite movies, but is also a rare occasion of a film adaptation being better than the novel it's based on.
Set at some point in the Victorian Era, Englishman Rudolf Rassendyl (Ronald Colman) pays a visit to the (unnamed in the film) country of Ruritania, whose king Rudolf V is to be crowned soon. As Rassendyl takes a nap in the woods in the middle of fishing, an interesting duo comes across him; Colonel Sapt (C. Aubrey Smith) and Fritz von Tarlenheim (David Niven), attendants to King Rudolf (also Ronald Colman) - who looks exactly like Rassendyl. It turns out that one of the king's ancestors had an affair with one of Rassendyl's ancestors some time ago, and thus the king has himself a perfect double. As it's the king's last night of freedom before his crowning, he invites Rassendyl to dine with him at his private hunting lodge. However, the king's half-brother, Duke Michael of Strelsau (Raymond J. Massey) has sent the king a bottle of wine which has been drugged, and the next morning, Sapt is unable to rouse him. Fearing that Michael will stage a coup in Rudolf's absence, Sapt persuades Rassendyl to impersonate the king at his coronation.