When the Doctor lands his TARDIS in the Lancaster town of Lannet in the present, he finds that something is terribly wrong. The people are scared. They don't like going out onto the streets at night, they don't like making too much noise, and they certainly don't like strangers asking questions. What alien force has invaded the town? Why is it watching barmaid Alison Cheney? And what plans does it have for the future of the planet Earth?