Henry IV is not the medieval German emperor of that name, but a twentieth-century Italian aristocrat who suffered a head injury in a fall from a horse during a costume pageant, in which he had been dressed as Henry IV. He went mad as a result, and his family arranged for him to be kept in a remote house by an entourage who play up to his fantasy, dressing the part and learning the history of the time. They find it easy enough, once in the swing of it; for Henry is every inch the emperor.
      Twenty years have passed when the action of the play begins. His family arrive with a psychiatrist, hoping to cure him. They have come only now because Henry's sister has recently died, leaving as her last wish a request that the family should try to help him. The psychiatrist forms a plan to shock henry back into his wits. But as the play proceeds, it becomes clear that Henry is no longer mad, but playing a part; love and jealousy, attempted murder, old conspiracies and feelings, enter into the complexity of his pretence, and are brought to a head by the family's visit and the open tensions between its members. Tom Stoppard has preserved the simplicity of the Pirandello's dramatic line, and enhanced its humour. The complexity exists in both the original and reorganised versions, though in Stoppard's version Pirandello's easy evocation of now-surpassed psychological theory comes to seem part of the joke. 
      No member of the cast allows the quality of performance to drop below the Donmar's perennially high standard, though Ian McDiarmid - acting superbly, credibly, movingly as the fantasy Henry IV - sometimes allows his voice to drop so low that he is completely inaudible to all but a few lucky folk in the front row towards which he happens to be turned. The part demands dynamic range: but not to that extent.
AC Grayling