"Things have to get worse before they can get better. Trouble is, I think they're about to get a lot worse. As worse as worse can be. And then some."

For centuries the Morphans have worked. With no help from other worlds, they subsist on the food they can grow and that's little enough. But their purpose, their whole life is to maintain the machines that will one day make their world as habitable as old Earth.

Life used to be hard. Now as their crops fail, livestock sickens, and the temperature drops, it's becoming impossible. This year's winter festival won't be the usual celebration. It's not a time for optimism or hope – and it's not a time to welcome unexpected guests. The Doctor, Amy and Rory find a society breaking apart under the strain. Tensions are mounting, old rivalries are coming to the fore, people are dying...

And then the Doctor's old enemies the Ice Warriors make their move. With the cold-hearted threat of invasion, the real battle for survival begins. Or does it? The Doctor begins to suspect that behind everything lies a deadlier, and even more chilling danger...