Henry IV, A Tragedy in Three Acts

A talented actor and historian falls off his horse in a historical pageant while playing the role of Henry IV. When he comes to, he believes himself to be Henry. For the next twenty years his nephew, Count de Nolli, funds an elaborate hoax in a remote villa, where actors play the roles of Henry's privy councillors and simulate the 11th century court.

On request from his dying mother, de Nolli brings a Doctor 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 in this one day of the visit. - Summary by Wikipedia