The synopsis for the episode reads:
One morning, in every city and town in the world, the human race wakes up to face the most surprising invasion yet.
Everywhere, in every land, a forest has grown overnight and taken back the Earth. It doesn’t take the Doctor long to discover that the final days of humanity have arrived…
"In the Forest of the Night" airs Saturday, 25 October at 8.20pm on BBC One and at 9/8 CST on BBC America.
[Source: Blogtor Who]