On the planet Chloris, metal is scarce, and without metal for tools, it is impossible to keep the planet's fecund jungle under control. When the TARDIS is forced to make an emergency landing on Chloris, the Doctor is captured by the power-hungry Lady Adrasta, who controls the very last metal mine on the planet, holding onto power through her Huntsmen and Wolfweeds. Romana is taken prisoner by rebel bandits, looking for some way of getting their hands...

Transmitting down from Nerva Beacon, the Doctor, Sarah Jane and Harry find themselves on Earth in the far future, long since abandoned by the human race. Soon it becomes clear that they are not alone: a shipwrecked crew of human astronauts are being hunted down, one by one. What is the creature that lives in the rocks, and what is the purpose of the lethal tests it is conducting? Can the Doctor ensure the safety of the Earth for the future generations...