Henry IV, Part 1 was a play by William Shakespeare featuring the eponymous King of England.
After Shakespeare inadvertently replaced King Richard III at the Battle of Bosworth and Richard took Shakespeare's place in 1597, the Fifth Doctor gave Henry IV, Part 1 as an example of a play which he was meant to start writing soon. The Doctor also suggested Richard could put his deceased brother George, Duke of Clarence into his first Shakespearean play to give him the immortality that history couldn't, an idea which Richard found appealing. In reference to Henry IV, Part 1, Richard asked "That alright, can I put him in that one?" to which the Doctor replied "Oh, I should think so". (AUDIO: The Kingmaker)
Behind the scenes[]
In the real world, George, Duke of Clarence did not appear in Henry IV, Part 1, which was set well before his birth. He did, however, make appearances in two other Shakespearean plays, namely Richard III and Henry VI, Part 3.
Although the title of the play as given in The Kingmaker implies the existence of a Henry IV, Part 2, it has never been explicitly mentioned in-universe. Additionally, Henry IV, Part 1 also features Falstaff and the future Henry V as characters, although their appearances in the play have not been confirmed by an in-universe source.