Trivia / Doctor Who S15 E2 "The Invisible Enemy"

• Acting for Two: In addition to voicing K9, John Leeson also voiced the Nucleus.
• First Appearance: Of K9.
• Inspiration for the Work: The story was heavily inspired by Fantastic Voyage.
• Prop Recycling: According to Matt Irvine, the model used for the cratered surface of Titan was a re-use of the model used for the surface of the Moon in Space: 1999.
• Similarly Named Works: Not related to The Outer Limits (1963) episode with the same title.
• What Could Have Been: The script was originally structured so that the scene in which K-9 departs with the Doctor and Leela could simply be dropped if the character was not to be retained, leaving the implication that he had simply been returned to Professor Marius off-screen.
• Working Title: The Enemy Within, The Invader Within and The Invisible Invader.
• You Look Familiar:
  • Michael Sheard (Lowe) makes his fourth of seven appearances in Doctor Who, having made previous appearances in "The Ark", "The Mind of Evil" and "Pyramids of Mars".
  • Frederick Jaeger makes his third appearance in Doctor Who, having previously appeared in "The Savages" and "Planet Of Evil"

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