The year is 2539. Arriving on Axista Four, the Doctor, Zoe and Jamie find the colony in a state of chaos. A breakaway group of colonists — the "Realists" — has abandoned Ransom's Back to Basics ideals and is creating a new high-tech settlement. The "Loyalists" who remain are dwindling in number and face total extinction. Who are the dog-like aliens who call themselves Tyrenians? What is the secret agenda of the sinister Federation Administrator Greene? And what really happened when the colony ship crash-landed on Axista Four 100 years ago?