A little boy goes missing; a policewoman begins drawing cave paintings; and the employees at the mysterious Glasshouse are desperate to keep everyone away -- the Doctor suspects it's all down to a group of homo reptilia. His assistant, Liz Shaw, has ideas of her own and has teamed up with a journalist to search for people who don't exist. Behind it all lies a conspiracy to exploit UNIT's achievements -- a conspiracy reaching deep into the heart of the British Government.
