Doctor Who: 15 Cool Details Peter Harness Just Revealed About The Zygon Invasion

15. Moffat Pitched The Story As A Sequel To The Day Of The Doctor

The Zygons were first introduced all the way back in 1975's Fourth Doctor story Terror of the Zygons, their sole appearance in Classic Who. After that, they weren't seen again until 2013's 50th anniversary special, The Day of the Doctor.

On Twitter, Harness stated that Moffat felt like he had "unfinished Zygon business" after penning the latter episode, which is partly what prompted him to suggest the idea of The Zygon Invasion and The Zygon Inversion to Harness.

In fact, Moffat pitched the story by using the phrase "sequel to Day of the Doctor", and the opening of The Zygon Invasion - which reminds audiences of the human-Zygon peace treaty that was established in the 50th - makes it abundantly clear that these two stories are essentially companion pieces.