“The Zygon Invasion” is episode seven of season nine of the new Doctor Who.
This episode opens with a narration by two Osgoods, related to the Zygon treaty negotiated in “The Day of the Doctor”. Following the treaty, twenty million Zygons have been resettled on Earth, all disguised as humans – one of them as an Osgood.
Unfortunately, one of the Osgoods – it isn’t certain whether it was the human one or the Zygon one – was killed by Missy in “Death in Heaven”. Then the treaty started to fall apart. One element of the Zygons has gone rogue, and kidnaps the surviving Osgood. They also appear to have declared war on the rest of humanity. With their ability to disguise themselves as human, it’s difficult to know who is friend and who is foe.
Osgood, before she was kidnapped, got a message out to the Doctor. Kate Stewart and UNIT are also involved, but they have a manpower problem. The Zygons may also have infiltrated more than suspected.
There’s an oblique reference to Harry Sullivan, the first new companion of the Fourth Doctor, who also encountered the Zygons. There also seem to be a number of contemporary political references hidden behind the human/Zygon conflict. There’s also another reference to hybrids, which seems to be the plotline for this season.
The story continues in the next episode, “The Zygon Inversion”.