'Explode the buoys? But that will destroy the Earth!'
'Oh dear, so it will. Pass on my apologies to the humans, won't you?'

Earth has been invaded. Twice. Thousands of years ago by a race searching for a new power source. More recently by the galactic marauders known as the Cat-People, who intend to continue the work done by the earlier visitors, with devastating results.

The recently regenerated Doctor, along with companions Ben and Polly, teams up with a group of amateur ghost-hunters and a mysterious white witch on a journey that takes them from twentieth-century Combria to the Arabian deserts of folklore and Australia 40,000 years in the past. Can the Doctor stop the invaders and disarm the bombs left buried beneath the planet's surface - or have the ancient Aborigines of Australia sung the seeds of their own destruction?