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.
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Trivia: When The Doctor puts on the gas mask and looks at the soldier who already has his mask on, The Doctor asks, "Are you my mummy?" This was a reference the "Empty Child" episode 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.
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