A talented actor and historian falls off his horse while playing the role of Henry IV in a historical pageant. After he comes to, he believes himself to be Henry. For the next twenty years, his wealthy nephew, Count de Nolli, funds an elaborate hoax in a remote villa, hiring actors to play the roles of Henry's privy councillors in order to simulate the 11th century court.
De Nolli's dying mother requests that he bring a Doctor, who is referred to as the latest in a succession to try to cure Henry (whose real name, if it is not Henry, is never mentioned). All the action of the play occurs on the day of the Doctor's visit.
Accompanying de Nolli and the Doctor are:
- Frida, de Nolli's fiancée.
- Frida's mother, Matilda (whom Henry loved, unrequited, before the accident). The character notes describe Matilda as widow, and neither Henry nor Belcredi are Frida's father. A portrait of the young Matilda in costume from the pageant, dressed as Matilda of Tuscany hangs on the wall of the throne room. Frida is now the spitting image of her mother as she was then.
- Matilda's lover, Belcredi.
In the first two acts the visitors play parts from the period whilst interacting with Henry.
The play begins with the induction of Bertoldo to the band of privy councillors. He has prepared for the part in Henry IV's court. The visitors then arrive and are later introduced to Henry. Henry mistakes Belcredi's disguise of a simple monk for Peter Damian and reacts angrily, but is later calmed.
Act two begins with speculation among the visitors about Henry, as well as what he sees in Matilda, who argues constantly with Belcredi. Henry enters once more and his behaviour is increasingly erratic. Once the visitors arrive Henry declares to his councillors that he is not truly mad, but has been aware of the nature of his existence for some time. However he has preferred to stay as he was than to live in the 20th century (the play is set in the 1920s). His behaviour and speech remain abnormal.
Upon learning of this revelation the visitors confront Henry, who acts angrily to them, particularly Belcredi. At the end of the act he grabs Frida, who is dressed as in the portrait in preparation for the Doctor's plan to shock Henry out of his madness. In the ensuing altercation Henry stabs Belcredi. The visitors flee, and Henry resumes his regal persona as the curtain falls.