Parts: 2 (45-minute)
Writers: Peter Harness and Steven Moffat
Airdates: 31 October — 07 November 2015
Production Code: n/a

Doctor: twelfth
Companion: Clara

Creature Feature: Zygons
Special Guests: UNIT

Story: On present-day Earth, following the resettlement and integration of 20 million Zygon refugees across the world, an uneasy truce maintains the peace between humans and Zygons... until a radical faction of Zygons, resentful about being forced to hide their true nature and live out their lives as humans, threatens to expose the cover-up by unmasking all of the transformed Zygons. Knowing that this would result in mass panic and a worldwide war, UNIT asks for the Doctor's help to de-escalate the situation.

Tracking down the Zygon splinter group takes Kate Lethbridge-Stewart to New Mexico, USA, while the Doctor investigates their training camp in the Central Asian country of Turmezistan. Clara, meanwhile, remains in London, where she becomes enmeshed in sinister goings-on underground...