The Christmas Invasion
The episode marks the return of the renegade Time lord known as the Master, who last appeared during the 1996 television movie. It also references the events of "The End of the World", "Boom Town", "The Parting of the Ways", "The Christmas Invasion" and "Torchwood" episode "End of Days". As well, it contains clips from "The Parting of the Ways", "The Christmas Invasion", "Human Nature" and "Gridlock" and audio clips of previous Masters when the Professor's watch is beckoning him to open it, including Anthony Ainley's laugh and a line Roger Delgado spoke in "The Dæmons".