New this week Trivia: When this episode and "Forest of the Dead" were made, only Steven Moffat and Alex Kingston knew who River Song really was. As a result, not only is the Doctor's confusion genuine, David Tennant famously admitted that he chose to act as if the Doctor thought that she was a future incarnation of his, even though he knew that, even with what little information was given on River in the story, that theory made no sense.
New this week Trivia: Early drafts of this episode had the Carrionite invasion in 1599 (from "The Shakespeare Code") and the Dalek invasion in 1930 (from "Daleks in Manhattan"/"Evolution of the Daleks") stopped by UNIT time-travelling commandos, but this was cut for time and being unnecessary to the main plot.
New this week Trivia: David Troughton, who plays Professor Hobbes, is the son of Second Doctor Patrick Troughton. He previously appeared in the classic series twice: alongside his father in "The War Games" as minor character Private Moor, and in the Third Doctor story "The Curse of Peladon" as King Peladon.
New this week Trivia: The UNIT soldier who is seen reporting that the Doctor is dead when Donna arrives at the scene the night of the Christmas Star incident is Private Harris, one of the soldiers who was mind-controlled by the Sontarans in the normal universe in "The Sontaran Stratagem"/"The Poison Sky."
New this month Trivia: The Doctor mentioning that someone tried to move the Earth once before, a long time ago, is a reference to the First Doctor serial "The Dalek Invasion of Earth", in which the Daleks intended to replace the planet's core with an engine in order to do the moving. And, of course, the Daleks are responsible for this episode's titular planet theft as well.
Trivia: When Harriet Jones is explaining the sub-wave network to the Doctor's companions she says "It was created by the Mr. Copper Foundation" This is a reference to Mr. Copper from 'Voyage of the Damned' who stayed to live on Earth and who probably would have had the advanced technology from the planet Stow where he came from.
Add timeProfessor Lazarus
Trivia: When the Doctor puts on the gas mask and looks at the soldier who already has his mask on, he asks, "Are you my mummy?" This was a reference to the episode "The Empty Child", where infected people wearing gas masks constantly say "Are you my mummy?"
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Trivia: As the Doctor and Donna stand in the room, Caecilius enters. He is played by Peter Capaldi, who later on took over the role of The Doctor from Matt Smith. It was later written into the storyline that the reason the Doctor's regeneration picked that face was as a reminder to himself that he saves lives. In addition, Karen Gillan, who later played Amy Pond, plays one of the Sybilline priestesses.
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