Based on Anthony Hope's 1894 novel, an Englishman substitutes as king to foil a plot to take over the kingdom, but he falls in love with the queen to be.
In Zenda Englishman Rudolph Rassendyll (Ronald Coleman) learns he resembles King Rudolph V (Ronald Coleman), who invites him to his hunting lodge. King Rudolph gets drunk; Col. Zaft (C. Aubrey Smith) reprimands him and is slapped. The king passes out. Zaft and Captain Fritz von Tarlenheim (David Niven) tell Rudolph that the king was drugged, and he must be crowned for a day, or Michael will take over. Zaft makes the woman in the wine cellar drink the drugged wine, and they lock up the king. Duke Michael (Raymond Massey) signs a proclamation of his regency and martial law, but Antoinette (Mary Astor) wants to marry him. Rupert of Hentzau (Douglas Fairbanks Jr.) cynically tries to woo Antoinette and tells her Michael plans to marry Queen Flavia.
Zaft and Fritz accompany Rudolph, who is crowned king to the consternation of Michael and Rupert. Princess Flavia (Madeleine Carroll) bows, and Rudolph kisses her. Flavia finds that Rudolph is behaving better, and Rudolph keeps Michael waiting. Flavia warns him about Michael and says she loves Rudolph as never before. Zaft takes Rudolph back to the lodge; but the king is gone. Rudolph invites Flavia to a ball and dances with her; but he puts off their wedding for six months. Flavia is disappointed, but Rudolph tells her he loves her. Rudolph tells Zaft he could stay king, but Zaft says he would have to be killed. Rudolph tells Zaft to find the king.
Rupert tells Michael that he has the king. Michael suggests that the Englishman could be killed first. Antoinette tells Rudolph he is in danger but makes him promise not to hurt Michael. Rupert offers Rudolph 50,000 pounds to leave and then shoots at him; but Rudolph escapes. Rudolph goes to the court of Strelsau. Flavia cries, and Rudolph says he has to go, asking her to do her part if he does not come back. Rupert calls on Rudolph at the lodge and tells him to attack the castle and let the others be killed so they can rule together. When Rudolph tells Zaft and Fritz, Rupert throws a knife at Rudolph and rides off. Antoinette cares for the king. Rupert laughs and tells the king he may be drowned. Michael asks the king to sign his abdication; but he prefers to die a king.
Rudolph gets a message from Antoinette, and he swims the moat and climbs in the window. Rudolph is given a gun and is instructed by Antoinette. Rupert finds Antoinette's door open. Michael sees Rupert with Antoinette and chokes him; but Rupert stabs Michael, who dies. Rupert kills the man opening the drawbridge. Rudolph fights two guards to rescue the king; but Rupert has a pistol. Rudolph offers him half the kingdom and knocks the gun away. Rupert and Rudolph fight with swords, and Rudolph cuts the drawbridge rope. Soldiers ride in, and Rupert jumps into the moat. King Rudolph asks Zaft to forgive him. Wounded Rudolph gives the king his ring back. Flavia knows all, and Rudolph asks her to come with him; but she says she was born to her duties. Rudolph says good-bye to Fritz and Zaft before riding away.
This adventure story has charm and grace; but with everyone speaking English in eastern Europe it is an unlikely fantasy that implies the Englishman is more virtuous and romantic.