The Doctor and Sarah 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.