The Doctor and Martha find themselves aboard a ship heading for a planet destined to become colonised by the humans aboard. However all is not well as they find themselves caught between two groups fighting for their survival from the others. One group calls themselves the pure humans, recently awoken from their cryogenic chambers, whilst the other is made up of the Artificials, a genetically engineered slave race whose job is simply maintenance. But it seems at first that the Artificial has used most of the ship's energy to duplicate themselves to overrun the ship.