The Doctor and Ace are in the spaceport of the planet Epajaenda. Ace has been chatting with security guard Essjee 78, while the Doctor has gone outside, which is forbidden, to take readings in the toxic atmosphere. He is looking for some kind of anomaly. The only other people in the spaceport are the other security guards, a blue tumbelweed creature, and a group of Travellers, frog-like beings.
Ace tells the Doctor what she has learned about the planet. It has been extensively mined for the last three hundred years, and is now completely gutted. It is about to be turned into a tip for industrial waste.
Essjee 78 tries to force the Doctor and Ace to board the last spaceship leaving Epajaenda, and Ace realises by his demeanour that he is an android. When Essjee sees that the Doctor has a dead Rat King in his bag, he and two other guards restrain the Doctor and Ace. A scream is heard from the tumbleweed creature as it is chased by several rats. The guards leave the Doctor and Ace to destroy the rats, and Crospa, spokesman for the Travellers, invites the Doctor and Ace into his tent.
As he serves them tea, he explains that they are natives of Epajaenda, but that three hundred years ago they met a man named Abraham Derris-Cuthbertson with whom they made a deal. He would provide spacecraft for them to leave their world, and they would allow him to mine resources from it. According to the terms of the contract, Derris-Cuthbertson is due to return the planet back to them, and despite the Doctor's scepticism, the Travellers have faith he will return.
Their attention is suddenly taken by a Traveller who has opened up the long-abandoned fountain in the spaceport. The Doctor realises this is the source of the anomaly. Several security guards, having finished off the rats, approach and try again to take Ace and the Doctor into custody. Ace almost fools them, and Crospa and his people try to keep the guards from taking Ace and the Doctor away.
At that moment, from outside the spaceport a giant globe of rats, called the Rat Emperor by the Doctor, break into the spaceport. The guards, though efficient shots, are overwhelmed. As the others try to get away, a spaceship approaches. The rats try to run away. Out of the spaceship comes Derris-Cuthbertson. He is here to honour his contract. He plans to use the anomaly to clean Epajaenda by channelling Dirrijali's oceans onto Epajaenda. The Doctor is angry at Derris-Cuthbertson's plundering of planets, but as he and Ace head for the TARDIS, the Doctor sees the Rat Emperor engulf Derris-Cuthbertson. Derris-Cuthbertson opens the anomaly.
- The androids work for Safensound, which is a subsidiary of the DC Mining Company.
- Ace mentions that the book How To Complain is in the TARDIS library. The Doctor thinks it must have been Tegan's.
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