The Fourth Doctor and Sarah Jane Smith are hiding from a giant mutant sheep behind a bush. Suddenly, a spear lands nearby, scaring off the sheep. A tribe of tall humanoids approaches them. A man, Euripides, tells them they are his prisoners, and they will be taken to the Temple of Life.
In the village, Sarah notices some electronics in the huts. Euripides leads them to the Elders' hut. Inside, they meet a man and a woman, the left-tenant and the right-tenant. The pair explain that they are threatened by another tribe who want the crucible, the source of life. Just then, the other tribe attacks, and the Elders retreat through the door to the crucible. The Doctor convinces Euripides to let him and Sarah in as well.
The crucible is a room full of stasis pods. The pods have long since failed and the bodies inside have decomposed. The Doctor realises they are on a spaceship like the Ark. The people have lived on it for generations and forgotten it is a spaceship. A humanoid drone being operated by the pilot appears, and takes the Doctor and Sarah to the bridge at gunpoint.
On the bridge, they find the pilot, Ana, in a state of decay, being kept alive by the ship. She has been alive for over three thousand years, piloting the first ark ship, Prosperity. The Doctor says he can end the mission by boosting the engines to reach a planet in a day, but it would destroy the engines and Ana would die.
On the TARDIS scanner, Sarah and the Doctor watch Prosperity land on a new planet. Then, the Doctor goes forward a thousand years and strolls outside to see how the colonists are getting on.
- The Doctor distracts the mutant sheep with his scarf.
- There are oak trees in the forest.
- In the time since Prosperity has left Earth, faster-than-light travel has been invented.
- This short story accompanied a chapter about space travel.