Harry Potter

The Doctor had read the Harry Potter series of novels, and cried upon reading the ending of the final book. Martha Jones also knew the books and suggested Potter's magic word "expelliarmus" to William Shakespeare to help defeat the Carrionites. The suggestion worked. (DW: The Shakespeare Code)

Behind the scenes

 * In 2005, Russell T Davies asked Harry Potter's creator J. K. Rowling if she would write for Doctor Who. She refused, saying that she was "was amused by the suggestion but simply doesn't have the time." She was writing the sixth Harry Potter novel at the time.


 * Before being cast as the Tenth Doctor, David Tennant played Barty Crouch Jr. in the 2005 film adaptation of Harry Potter and the Goblet of Fire. In The Christmas Invasion, a Hogwarts-style uniform briefly appears as the Doctor considers what clothes to wear in his new incarnation.


 * The Shakespeare Code, in which The Doctor references reading the final book, was broadcast before the book was published, during a period of intense anticipation for its release. The Doctor doesn't mention the book's title, as it had not been revealed by the time the episode began filming.