A long time ago, the Doctor made a deal in the Tower Of London. 20 million Zygons walk among us, in human form, living undetected in peace and harmony. But cracks are showing in this delicate peace. Humans and Zygons are disappearing. In city apartment blocks, lifts are going missing, and far below the streets of Britain, alien pods are growing in secret caverns. Unit's scientific advisor, Osgood, sends a desperate message to the Doctor - but since Osgood is long dead, how is that even possible?