Able to metamorphose into replicas of other living organisms and fire electrical charges from their hands that can stun or disintegrate those it touches, the Zygons were stranded on Earth centuries ago. The Zygons have a mission to take Earth as their new home after Zygor, their homeworld, was destroyed in The Time War.

The Doctor brokered a perfect peace between the Zygons and Humanity, wiping their memories of who was Zygon and who was Human. With millions of Zygons living on Earth in the guise of ordinary Humans it can only be hoped the fragile peace holds or there will be truth or consequences…