Henry IV thinks that he is living during the eleventh century. Every effort is made to encourage his delusion. In fact, he lives in the present day.

The experienced valets introduce Berthold to his new position as a secret counselor at the court of Henry IV. Count Charles Di Nolli, nephew of Henry IV, hires these men to dress in eleventh century costumes and impersonate the participants in the historical debate between church and state that took place in Canossa in 1077. The valets complain that, though dressed and ready, they are idle until Henry IV supplies them with a cue to act. The men examine two portraits depicting a youthful Henry IV and his archenemy, Matilda, marchioness of Tuscany, that are stylistically at odds with the medieval surroundings.

Charles, accompanied by the marchioness Dona Matilda Spina, and her daughter, Frida, who is engaged to Charles, bring Doctor Dionysius Genoni to observe and evaluate Henry IV’s condition. Baron Tito Belcredi, Matilda’s lover, completes the party. Matilda’s likeness served for that of the portrait of Matilda of Tuscany, so Charles orders the valets to lock Henry IV’s door to keep him away from her. The doctor expresses surprise that the surrounding illusion is so complete, while the others are surprised that Matilda’s twenty-year-old portrait resembles Frida more than Matilda.

The portraits, souvenirs of a pageant twenty years earlier, remind everyone of a tragic occurrence. No one remembers who first proposed the pageant. Belcredi insists that he originated the idea, but Matilda thinks not. She does reveal, however, that her choice to represent Matilda of Tuscany causes Henry to impersonate Henry IV, in order to be near her. She also confesses that he had been in love with her, which causes Belcredi to dispute the true character of Henry IV. He has been a superb actor, so is he really outgoing and humorous, or has he simulated it? Belcredi sees Henry IV as his rival for Matilda’s affections and, though no one knows it, he pricked Henry IV’s horse during the pageant, causing Henry to fall and strike his head. When Henry regains consciousness, he thinks he is the historical figure.

Berthold rushes in. He unintentionally angers Henry IV and fears reprisal. His entry agitates and frightens the visitors. The valets explain that it is no longer possible to isolate Henry IV so the doctor, Belcredi, and Matilda decide to don costumes and interview him. Matilda chooses to be the mother-in-law of Henry IV. The doctor chooses to be a bishop. Belcredi chooses to be a monk. It almost seems that Henry IV penetrates their disguises because he accuses Belcredi of being Peter Damiani, another archenemy of Henry IV. Henry IV’s appearance, including dyed hair and makeup to retain the look of youth in the portrait, surprises the others. At the interview’s conclusion, Henry IV bows, leaves the room, and Matilda bursts into tears.

Later, as the Doctor and Belcredi discuss Henry IV’s condition, Matilda insists that he is not insane. Matilda sends for her old masquerade dress. The doctor plans to shock Henry IV into sanity by placing Frida, dressed as Matilda, and Charles, dressed as Henry, into the empty portrait frames. As they seem to come alive, Matilda is to appear as herself to usher Henry IV into reality.

Frida enters wearing the costume, and they all experience déjà vu. Landolph asks the doctor and Matilda to put their costumes on again and see Henry IV. The others leave, and Matilda tries to convince Henry IV that Matilda of Tuscany is interceding with the pope in his behalf because of his love for her. Alone with his attendants, Henry IV, suddenly furious about his wasted years and Belcredi’s liaison with Matilda, admits that he recovered his senses sometime earlier. The valets do not know whether to believe him, but he is able to convince them.

Henry IV says good night to his attendants. As he walks down the darkened hall, the doctor’s plan is put into effect, and Henry is badly frightened as the images seemed to come to life. Frida, timid, is frightened in turn, and everyone rushes in from their hiding places. Matilda has heard of Henry IV’s recovery from the valets. Everyone is angry at his ruse. Henry IV tells of his gradual recovery and justifies his deception by explaining that he has lost so many years of his life that he prefers to continue wearing the mask. He remembers Belcredi causing his accident and knows why he did it. In his mind, Frida is the Matilda he remembers. He terrifies Frida when he takes her in his arms.

When Belcredi rushes at him, Henry IV draws Landolph’s sword and stabs Belcredi. Seriously wounded, Belcredi is carried out, insisting that Henry IV is sane. A moment later, Matilda screams her grief, and Henry IV realizes that, by his rash action, he has condemned himself to a lifetime’s impersonation of Henry IV.