The Doctor and Peri arrive via TARDIS near a small medieval village and a nearby castle. They find a young man named Gurth being chased down by what appears to be Herne the Hunter and his dogs. To the Doctor's curiosity, the dogs appear to be robots. After evading Herne, Gurth explains that he was being hunted as his "time has come", and that he is no longer welcome in his village. He also tells them that he knows of no lands outside of area that he can escape to. They encounter a group called the Pariahs, similar outcasts like Gurth that have evaded Herne and hidden away in a nearby cave system. The Doctor continues to suspect something is amiss when he is captured by the Duke that rules from the castle, finding that he has no master outside of a disembodied voice that he answers to, and that the castle is unlike he has seen. The Doctor eventually escapes, reunites with Gurth and Peri, and explore more of the Pariahs' caves. They come to a metal hatch, and on the other side a small control room, through which they see the vast expanse of space. The Doctor confirms his theory: they are aboard the Leviathan, a giant generational colony ship used by humans, explaining why no one aboard knew of this. The Duke is answering to the ship's automated computer system that is trying to maintain the systems, but failing like much of the rest of the ship. The Doctor surmises that this was a ship belonging to the Sentinels of the New Dawn, a 22nd century force that used the Herne imagery. Outside of the ship, they see a much smaller ship staying in close proximity to the Leviathan but cannot contact it. When they leave the caves, they learn that all the adults of Gurth's village are rounding up the children to take to the castle. The Doctor and Peri help the children to escape before traveling to the castle themselves to help free the captured Pariah. The Doctor finds under the castle's foundation numerous cloning tanks, and determines that the Sentinals had planned to keep cloning themselves until they arrived at their destination. However, the damage to the computer systems caused them to forget this, and instituted the process whereby when a clone child got old enough to start to question their existence, they were then considered expendable and hunted down by the Herne, while a new clone replaced them. All the adults of the villages, the Duke, the Herne, and the castle guards are all robots doing the program's bidding. The recent change was an order issued from the other ship, who have access to the Leviathans computer systems. The Doctor is able to coerce the computer systems to stop its actions before anymore bloodshed befalls them. One of the crew of the smaller ship is able to transport onto the Leviathan, warning that his comrades are more interested in tearing down the Leviathan for scrap and are trying to kill the living humans to do so. The Doctor teleports back to the smaller ship and is able to stop them. With the Doctor's help, they arrange for Earth to send a rescue vehicle to evacuate the humans from the Leviathan.
