sacrifice commences, a second party of Daleks arrive, which have replaced their energy weapons with mechanical firearms that use bullets. The Daleks kill a number of Exxilons, then force the Exxilons and humans to mine Parrinium. The Doctor and Sarah Jane flee into tunnels. The other party of Daleks arrive and they discuss their actual plan to use Parrinium to create a plague and spread it across every planet except their own. The Doctor and Sarah Jane meet a group of subterranean, fugitive Exxilons. Their leader, Bellal, explains that the city was built by the Exxilons' ancestors, who were once capable of space travel. They built the city to be capable of maintaining, repairing, and protecting itself. Fitting the structure with a brain meant that the city no longer needed its creators. On realising this, the Exxilons had tried to destroy the city, but the city destroyed most of them; the savage surface dwellers and Bellal's group are the only survivors. Bellal's people seek to complete their ancestors' last, failed act – to destroy the city and ensure their race's survival. Bellal sketches some of the markings on the city wall; the Doctor recognises the markings from a temple in Peru deducing that the ancient Exxilons visited Earth. Bellal also explains that the city supports itself through underground "roots" and the aerial beacon. The Doctor realises that the beacon must be the cause of the energy drain and decides to go to the city and resolve the problem. The Daleks come to the same conclusion and create two timed explosives to destroy the beacon. One Dalek supervises two humans placing the explosives, but one of the humans, Galloway, secretly keeps one bomb. Two other Daleks enter the city to investigate the superstructure, but the Doctor and Bellal enter the city just before them. The two parties then proceed through the city, passing a series of progressive intelligence tests. The Doctor reasons that the city has arranged the tests so that only lifeforms with knowledge comparable to that of the city's creators would reach the brain, allowing the city to add the knowledge of the survivors to its databanks. On reaching the central chamber, the Doctor begins to sabotage the city's computer brain; the machine responds by creating two Exxilon-like "antibodies" to "neutralise" the Doctor and Bellal. The pair are saved when the Daleks enter and fight the antibodies, and the