The TARDIS materialises and the Doctor is amazed to find a pig's head filling the scanner screen. A closer look shows it is one of several dishes being carried by a spherical robot. After checking the environment is safe, he emerges to find himself in the wide corridor of a palatial building, hung with elaborate tapestries. He follows the smell of food and emerges on a gallery above a banqueting hall where men in metallic tunics feast.
Suddenly, he notices a black-clad figure in the gallery aiming a gun at one of the diners. He disarms the would-be assassin with his stick and the man flees. The diners send a robot to bring him down forcibly and accuse him of being a spy for the Crustians. Calling themselves the Sons of Grekk, they disbelieve his assurances that he is an offworld traveller, and mock the TARDIS when he claims it is his vessel. The eldest, Swarf, urges caution, believing it dangerous. He tells his brother Morag to take the Doctor to the Pit. The robots, called Mechanislaves, are set to guarding the TARDIS.
The Pit proves to be a smooth-sided volcanic crater, in which members of various sentient species are imprisoned. Morag pushes the Doctor in. His fall is broken by webbing spun by large, spider-like prisoners. Other prisoners resemble cockroaches, butterflies (their wings clipped) and turtles. These last are the Crustians; one of these explains the black-clad assassin is an evil genius named Deemon, left on the planet by Grekk to provoke his sons to endless hostility towards its other inhabitants. He has also smuggled a sustenance unit into the Pit, which is keeping the surviving prisoners fed.
The Doctor converts the unit into a radio transmitter, with which they tell the other Crustians of the Pit's location and ask for rescue. The Sons of Grekk intercept the message, and venture into the Pit to massacre its occupants before the Crustians arrive, but the Doctor has organised a trap using the webbing of the spider-creatures. The Sons are wrapped up. As the prisoners leave by the ladders by which they descended, they are left to cool off in the Pit and change their minds about the need for peace.
The Doctor accompanies some of his fellow escapees to the Sons' castle where Deemon has programmed the Mechanislaves to be hostile; but the Doctor bluffs that his stick is a gun, and orders Deemon to stand them down. He notices to his horror that another party of Mechanislaves are about to throw the TARDIS from a high window. He grabs the lower edge, hauls himself up, opens the door, slides across the control-room floor and dematerialises the TARDIS before it hits the ground.
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- The Doctor is described as "Dr Who" throughout.
- Although the text does not make it clear how many sons Grekk has, an illustration shows four of them.
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