The Doctor and Heather respond to a distress signal. It comes from the Nurburr Asteroid, a small rock in space housing a robot making factory. It is protected by a Tisk forcefield dome secure enough to keep out even a Dycoatean jelly flea. They meet Watnul, the asteroid's the only inhabitant. Having lived alone on the asteroid for years, he continues his centuries-old family business of building the most intelligent serving robots in the universe.