Against a background of local skirmishes between rebel forces and government troops in an unnamed Asian/Indian country, Philip and Shanya meet on the train, travelling to visit their respective scientist fathers in a camp in the desert, below the ruins of a very ancient city. The scientists are bargaining with the authorities to have water piped to the ruined city. But after their first night, the children wake to find the camp deserted. When, after a day, still no one returns, they make for the distant city just visible in the mountain. In fact the city is nothing but ruins, once rich with gardens and fed by streams, but now bleached and dry, and utterly deserted - or are they? There is a sensation of presences, footsteps, a belief they are being watched. They come upon a monkey, toasting a lizard on a fire. It turns and speaks to them - a strange mixture of muddled English, sometimes with the voices and the intonation of their fathers. In fact the ancient city is peopled by a whole tribe of these talking apes - the secret subject of the archaeological camp's study. But the apes too are in trouble - their city is dying from the drying up of wells and water courses. And children and apes together are now forced to flee into caves to escape rebel forces who bombard and occupy the city. In the ensuing journey underground through the heart of mountain they forge new alliances, water is restored to the ancient city, and the animal garden can once again sustain life...