Separated from the Doctor, Clara discovers a new menace from another dimension. But how do you hide when even the walls are no protection? With people to save and the Doctor trapped, Clara comes up against an enemy that exists beyond human perception.
One morning in London, and every city and town in the world, the human race wakes up to the most surprising invasion yet: the trees have moved back in. Everywhere, in every land, a forest has grown and taken back the Earth.