UNIT attempt to track down the Zygon ship while Broton prepares to have the Skarasen attack London.
Quest roles:
Ian Marter(John Andrews), Nicholas Courtney(Brig. Lethbridge-Stewart), John Levene(Sergeant Benton), John Woodnutt(Seron), Lillias Walker(Sister Lamont), Ronald Gough(Zygon), Robert Russell(The Caber), Keith Ashley(Zygon)