Clara (Jenna Coleman), Danny Pink (Samuel Anderson) and some of their students at Coal Hill School, on an overnight trip, wake up to find that Earth has been covered by forest. One of the students, Maebh Arden (Abigail Eames) hears Clara thinking about the Doctor (Peter Capaldi) and finds him in Trafalgar Square. The teachers and students regroup there. In the TARDIS, Danny finds a pile of notebooks Clara left behind and realises that Clara lied about no longer travelling with the Doctor. Maebh’s notebook is among them, filled with pictures of an angry sun killing trees. Maebh goes missing and Clara reveals that Maebh has been taking medication to calm the voices in her head ever since her sister Annabel disappeared.

The Doctor believes a giant solar flare will strike the Earth today. They find Maebh, and the Doctor thinks she is communicating with something. He creates a gravity field around her, revealing firefly-like creatures. They speak through Maebh, claiming that they grew the forest. The Doctor offers to take Clara, Danny, and the children away in the TARDIS, but Danny wants to stay with the children and Clara doesn’t want to be the last of her kind. The Doctor thinks that the creatures were referring to the Tunguska Event and the Curuca Impact, which should have been catastrophic for all life, but the trees shielded Earth. The Doctor hacks into the Global Cellular Network and Maebh reads off a message telling everyone to leave the trees alone and to ask Annabel to come home. Danny wants to know the truth about Clara’s travels. The Doctor and Clara watch the now harmless solar flare. Humanity will forget about the forest, as they always have, but the memory will linger as fairy tales. Annabel appears to Maebh.

The episode refers to a number of fairy tales, as well as Blake’s 1794 poem The Tyger. There is a tiger and a wolf, but they are not the best of special effects. It received mixed reviews, some thinking it was lacking in threat. An advertisement on a bus features characters reminiscent of the Doctor and Clara. The bus was a cardboard cut-out because it was too difficult to get a double-decker bus into the forest. There has always been an entertaining, if awkward, tension between Science Fiction and Fantasy in Doctor Who, and this, like Kill the Moon, is an example.