Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Jenna Coleman (Clara Oswald)
Samuel Anderson – Danny Pink
Abigail Eames – Maebh
Jayden Harris-Wallace – Samson
Ashley Foster – Bradley
Harley Bird – Ruby
Michelle Gomez – Missy
Siwan Morris – Maebh's mum
Harry Dickman – George
Jenny Hill – Herself
Eloise Barnes – Annabel
James Weber Brown – Minister
Michelle Asante – Neighbour
Curtis Flowers – Emergency services officer
Kate Tydman – Paris reporter
Nana Amoo-Gottfried – Accra reporter
William Wright-Neblett – Little boy
Directed by Sheree Folkson
Written by Frank Cottrell-Boyce
Script editor David P Davis
Produced by Paul Frift
Incidental music composer Murray Gold
Series Series 8
Length 45 minutes
Originally broadcast 25 October 2014
Clara Oswald and Danny Pink chaperone several Coal Hill students on an overnight stay at the London Zoological Museum, and wake up to find all of London and the rest of the world have been covered by large forests. One of their students, Maebh, wanders off, but is found by the Doctor, who has landed his TARDIS in Trafalgar Square. Clara, Danny, and the other students regroup with him to recover Maebh and to try to understand what is going on, but the Doctor is unsure. Danny finds a pile of student notebooks aboard the TARDIS that Clara had accidentally left behind, and he realises she lied to him about forgoing future travels with the Doctor. Among the notebooks is Maebh's, each page having a picture of an angry sun striking down trees. Clara explains that after the disappearance of her sister Annabel, Maebh has exhibited strange tics and heard voices in her head, and has been on medication to calm these effects.
When Maebh wanders off again, the Doctor and Clara set off to find her, shortly followed by Danny and the other students. The Doctor explains that he believes a giant solar flare, similar to the one that destroyed the Bank of Karabraxos ("Time Heist"), is due to strike Earth that day, and that Maebh is central to the appearance of the trees. They find Maebh safe, but are soon surrounded by loose animals from the London Zoo, freed when their cages were torn asunder by the trees. Danny warns off the animals and rescues the three. As Maebh's last medication wears off, the Doctor examines her movements and thinks she might be communicating with something. He temporarily creates a gravity field around Maebh, revealing a number of glowing bug-like creatures. They speak through Maebh, explaining they have been on Earth since the dawn of time, and were called to Maebh after the disappearance of her sister. They claim responsibility for growing the forest, as they had done before in the north and in the south. The Doctor dispels the field when he finds the creatures are in pain due to it.
The Doctor mulls over this and regrettably believes that the Earth is doomed from the solar flare, and offers to take Clara, Danny, and the students safely away. Clara and Danny agree to stay on Earth with the students, and Clara tells the Doctor he should escape on his own given how many times he has saved the planet in the past. As the Doctor watches from orbit, he is stuck by inspiration and returns to Earth. He tells Clara the creatures were referring to the Tunguska Event and the Curuçá impact, events that should have been catastrophic for life on Earth. The Doctor believes that the trees were created to buffer the effects of the solar flare as they had for these events. Danny reports that the world governments, unable to burn down the trees due to their ability to control oxygen near them, are planning on using defoliating agents on the trees, which would render them unable to stop the solar flare. The Doctor hacks into the global cellular network and Maebh reads off a message prepared by the other students to warn the governments off this plan, and to request Annabel return to her. The governments stop their plan in time. As Clara and Danny escort the children home, Danny tells Clara he knows she lied to him about her travels with the Doctor, but forgives her and allows their relationship to grow.
Later, the Doctor and Clara watch in orbit from the safety of the TARDIS as the solar flare harmlessly strikes the Earth. This event is also watched by Missy on her tablet, calling the result a "surprise". The Doctor returns Clara to Earth as the trees start to dissipate; he explains that humanity will forget about the sudden appearance of the trees, as they have from the past events, but the memory will linger as part of fairy tales. Elsewhere, as Maebh meets with her mother outside their house, the last traces of the trees disappear, revealing Annabel hiding behind it. The family is happily reunited.