In the episode, a peace treaty agreed between humans and shapeshifting aliens called Zygons in the 50th anniversary special "The Day of the Doctor" has started to fail.
In the present, Osgood is captured by a splinter group of Zygons in the town of Truth or Consequences, New Mexico just after sending a warning to the Twelfth Doctor that the treaty is failing.
The Doctor and Osgood bring a Zygon, injured from a bombing run, aboard their flight back to the UK.
In New Mexico, UNIT leader Kate Stewart finds Truth or Consequences uninhabited aside from the sheriff, Norlander.
[2][3] Kate references Harry Sullivan, a companion to the Fourth Doctor in this serial, as the "naval surgeon" that helped create the Z67 anti-Zygon nerve gas.
Kate states the gas was developed in secret at Porton Down, alluding to Harry's location as reported by the Brigadier in the serial Mawdryn Undead (1983).
Both have shirts with question marks on the collar points, a common element of the Fourth, Fifth and Sixth Doctors' outfits.
This continues a running joke from earlier in the series: Prime Minister Harriet Jones is regularly told this after introducing herself, multiple times in both "The Christmas Invasion" (2005), and in "The Stolen Earth" (2008).
[4][3] Clara claims to memorize obscure facts to help her win at Trivial Pursuit, a popular trivia quiz board game.
[4] The episode pays homage to several science-fiction classics, including Invasion of the Body Snatchers and The Midwich Cuckoos.
[13] When the Doctor enters the presidential plane he raises both of his hands showing the victory sign, a pose commonly associated with former US president Richard Nixon.
The site's consensus reads "With "The Zygon Invasion," Doctor Who delivers a thrilling episode that pays special attention to character development and the consequences of time travel".
[25][26] Tim Martin of The Daily Telegraph awarded the episode four stars out of five, particularly praising Osgood's characterisation as "Earnest, resourceful and an unabashed superfan".
Calling the episode a "classic", she further said that it "more than lives up to the hype, with some stellar political commentary, brilliant performances and some very creepy child actors".
She further praised the episode's political themes by saying "The parallels between immigration debates aren't exactly subtle here, but they're so well-drawn it's impossible to mind".