In the early 19th century in the Moroccan desert, brigands attack the small caravan of ambassador Don Felipe Castro and his wife Dolores. The assault is countered by the arrival of the sultan's cavalry, led by Capt. Carlos Delargo, who escorts the ambassador's group safely back to the sultan's camp. That evening, Castro inquires about Carlos, confirming that he is the son of the king's former captain of the guards, who ran away with the king's sister and was sent into exile. Later, Carlos is followed to his tent by Dolores, with whom he had a relationship months earlier in Tangiers. The couple is interrupted by the suspicious Castro, who challenges Carlos to a sword fight, during which a lamp sets the curtains ablaze. Carlos fatally wounds Castro, but while struggling to extinguish the flames, fails to prevent Castro from shooting Dolores before he dies. Carlos is then sent to the palace of King Lorenzo III of Mandorra for trial, but Prime Minister Triano interrupts King Lorenzo's evening with Countess Flora to advise him not to try Carlos in public, as his features are identical to those of the king. Lorenzo agrees to announce Carlos' execution publicly while allowing him to escape to Portugal. Meanwhile, Lorenzo's cousin, Prince Ramon, plots with a French representative, Monsieur De LaForce, and Maj. Schrock to prevent the upcoming nuptials of Lorenzo and Princess Teresa of Corona, which have been engineered to seal their nations' alliance. To that end, Ramon plans to have Lorenzo use a sabotaged rifle during the following day's royal hunt. That evening, Lorenzo assures Flora that despite the necessity of the wedding to Teresa, he remains devoted to Flora and pledges to engrave the promise on her locket. The next day at the hunt, Lorenzo is severely injured while firing the altered rifle but is discovered by Capt. Ruiz, who carries him, unconscious, back to Triano. After Lorenzo receives treatment from Dr. Lopez, Triano and Ruiz agree to keep the king's injury secret, believing that Ramon must be behind the attack. When word comes that Princess Teresa is arriving earlier than expected, Triano sends Ruiz to intercept Carlos and moves Lorenzo to the deserted arsenal building. Ruiz sends Carlos to the palace, where Triano pleads with him to assume the king's place just long enough to greet Teresa and confound Ramon. Carlos warily agrees and is hastily attired and advised of protocol for the princess' reception. At the reception, Ramon and De LaForce are stunned when "Lorenzo" appears, and Carlos finds he is attracted to Teresa, who, aware of Lorenzo's feelings for Flora, is surprised by his sincere attentions. When Triano warns Carlos not to engage Teresa because she will be hurt when Lorenzo returns, Carlos agrees and spends the remainder of the evening with Flora. Suspicious, Ramon and De LaForce meet Flora later and ask her to learn Carlos' true identity. Flora visits Carlos and realizes he is not Lorenzo when he fails to recognize her locket and his vow. She pleads to learn of Lorenzo's whereabouts, but Carlos admits he knows very little. After Flora cautions Carlos that Ramon is behind Lorenzo's attempted assassination, she returns to the prince insisting that Carlos is really Lorenzo. Convinced that Flora is lying, Ramon and De LaForce invite Carlos to a bachelor party, but take him instead to the royal crypt, where De LaForce and Schrock torture him to reveal Lorenzo's whereabouts. The following day with the betrothal ceremony two hours away, an alarmed Ruiz informs Triano that Carlos has been missing all night. Meanwhile, Carlos feigns unconsciousness and when untied, manages to escape. At the palace, Teresa waits with increasing anger for the king to appear at the ceremony while Ramon gloats. When Carlos finally arrives, the ceremony continues, to Ramon's dismay. In exchange for her support of Carlos, Triano takes Flora to a recovering Lorenzo, who is determined to have his cousin hanged. Ramon, meanwhile, attempts to get Dr. Lopez to confess Lorenzo's location until one of the major's associates, Carnot, reports that Flora was followed to the arsenal. Ramon and De LaForce use Lopez as a means to enter the arsenal, then kill the doctor and the guard. With Lorenzo recovering, Triano pays off Carlos, who attempts to apologize to Teresa, who has been apprised of the situation by the prime minister. De LaForce arrives at the castle to announce that Lorenzo is being held by Ramon and his men at the arsenal. When Triano refuses to consider Lorenzo's abdication, Carnot, waiting outside, rides back with the decision. Carlos and Ruiz race after Carnot and enter the arsenal through a secret passageway. When Ramon cannot force Lorenzo to abdicate, he threatens to kill him, but instead stabs Flora, who has intervened. Carlos arrives and duels with Ramon and Schrock. When Lorenzo weakly attempts to help, he is run through by Ramon before Carlos kills him. Before dying, Lorenzo thanks Carlos for his service. Later, Carlos refuses to continue in the role of Lorenzo, until Teresa points out that he is the last of the royal line and she would be happy to remain his wife.