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 Ransome'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. Meanwhile, a spaceship from Earth has arrived with news that 80,000 refugees are about to descend upon the planet; the Realists are staging raids on the wreck of the colony ship, and in a secret underground bunker mysterious aliens who claim to be the planet's first colonists are beginning to awake. 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?