"...the party of explorers, at the extremity of their strength, had the delusion that there was one more member than could actually be counted."It's the year 2128. In cooperation with the Global Mining Corporation, the New Earth Government has sent 5000 colonists to Earth's first interstellar colony: Proxima 2. This first team of colonists are there to build the city in preparation for the second wave of colonists who will arrive in approximately another eight years. The colonists are cut off from earth—which is fine. Governor Percival is keeping order in the city, people are working hard, and aside from a few minor accidents, nothing bad has happened.Until, that is, the murders start. A man named Leary has apparently gone mad after a reasearch trip into the mountains. The Doctor and Sam arrive (Sam still worrying if there are still nanobots inside her) just as the bodies are beginning to add up. The Doctor quickly figures out what's happening, and its not Leary. An evil force lives in the mountains and can change its face to mimic that of the colonists—in this case, Leary. If the Doctor can't prevent Percival from destroying her own city in her attempt to retain order, and stop the creature in the mountains, earth's first colony may yet go the way of Roanoke.
— From an account of the early Antarctic explorers
- Emergency Presidential Address: Percival gives one after all the violence, then turns on Sam and Leary.
- Impersonation Gambit: The evil in the mountains a face-changer. It wants Leary to be blamed for everything.
- Police Brutality: De Winters, the head of the Installation Security Police is extremely brutal - just like his boss.
- Settling the Frontier: the colony on Proxima 2 is Earth's first interstellar colony
- Taking You with Me: Cheeky Monkey's species will not survive if he destroys the Face-Eater but he chooses to do so anyway to rid the world of the evil.
- They Look Like Us Now: The realization that the Doctor has that the murderer is not, in fact, a human, but simply an alien morphed into one.
- Ultimate Life Form: The Face-Eater believes that he (she? it? they?) is this.