When the TARDIS lands in the sleepy English village of Devesham, Sarah Jane thinks the Doctor has finally got her back home. But all is not as it seems - the village is unusually deserted and deadly white-suited spacemen patrol the countryside. When the dead begin to come back to life, the Doctor decides to contact UNIT. But their UNIT friends are also dangerously changed. Who is Senior Defence Astronaut, Guy Crayford, and why is he in charge of UNIT? Who are his true masters, the Kraals, and what insidious plans are they concocting behind the scenes? The Doctor soon discovers the invasion of Earth has already begun, and if he doesn't stop it mankind will be utterly wiped out.