Maebh

Clara Oswald and Danny Pink are chaperoning a small number of children from Coal Hill School on a overnight stay at London's National History Museum. However, when they awake the following morning they discover that London, as well as the rest of Earth, has suddenly been taken over by forests of trees in a matter of hours.

The Doctor also learns of this when one of the children, Maebh, finds his TARDIS in a forest-covered Trafalgar Square. When Clara phones The Doctor, to inform him of what has happened, she is initially saddened to find that he is already aware. The Doctor then reveals to her that it was one of the children that she and Danny are responsible for is with The Doctor instead of in the museum. This causes Clara to rush back to the museum to inform Danny that they have 'lost' one of the children. But when Danny discovers that Clara contacted The Doctor, before the children's parents, he reacts resentfully upon finding out Clara is still secretly keeping in contact with The Doctor.

Clara and Danny then lead the group to Trafalgar Square to be reunited with Maebh and The Doctor. There they talk about the trees and The Doctor suspects that the only way the trees could have grown so fast is due to the influence of time. They all enter the TARDIS and as The Doctor works with the controls Danny finds Maebh's notebook which Clara had accidentally left aboard during her last trip in the TARDIS. The notebook is filled with pictures of an angry sun striking down trees. Clara explains that Maebh's sister Annebel had disappeared a year prior leaving Maebh traumatised, and has been on special medication to silence the voices in her head. The Doctor realises that Maebh is central to the appearance of the forest, the condition allowing her to tune to some other communication frequency. When they realise that Maebh is not with the rest of the children The Doctor and Clara race out to find her.

The Doctor

They find that Maebh has dropped personal items along the path to allow her to be followed. They encounter a government service team, under COBR orders, trying to burn a path through the forest but the trees are able to control the oxygen nearby to prevent them from being burnt. As they continue following Maebh, The Doctor explains to Clara that Maebh's drawings show a giant solar flare, similar to the one that destroyed the Bank of Karabraxos (see "Time Heist"), that he confirmed is occurring presently and is due to strike the Earth that day. The Doctor though is curious as to how Maebh could predict this in her drawings.

As they continue after Maebh, they hear sounds of wild animals, which The Doctor believes have escaped from London Zoo - its cages torn asunder by the trees' growth. They manage to reach Maebh just in time to rescue her from several wolves that have been chasing her. But they realise too late that the wolves themselves were being chased by a tiger which then starts to threaten The Doctor, Clara and Maebh.

Clara

Danny though arrives with the rest of the children and scare off the tiger with their torches. As they regroup Maebh runs off again, waving her hands wildly, which the other children say is a tic that she has had since her sister died. Catching up with her, Maebh explains that she thinks she created the forest from a dream she had after her sister's death. The Doctor is able to interpret her ranting and motions and adjusts the local gravity near her with his sonic screwdriver, revealing a number of glowing bug-like creatures that circle Maebh. They speak through Maebh, explaining they have persisted throughout time and were called to the present by Maebh. They cryptically describe how they created the forest as they had done before. When the beings express pain, while under the field of higher gravity, The Doctor lets them go and Maebh collapses.

Learning all this The Doctor reveals to the others that the Earth is truly doomed by the solar flare, and Clara suggests they return to the TARDIS so that they can at least save the group of children. However, when they get to the TARDIS, Clara insists The Doctor leave on his own, refusing to abandon Danny and unwilling to be the last human left alive, and that for all the times The Doctor has saved the Earth, it is time for Earth to allow The Doctor to escape.

After some persuading The Doctor regretfully takes off in the TARDIS. But as he monitors the solar flare leave the sun inspiration hits him and he quickly returns to Earth and catches up with Clara and the others. He explains that he has interpreted the cryptic message from the beings, and has determined that the trees have actually saved the Earth before - That they were responsible for protecting the Earth from disaster during the Tunguska Event and the Curuçá impact by rapidly growing trees across the planet to buffer the effects of these impacts that should have otherwise extinguished all life on Earth. The sudden appearance of the forest now is to prevent damage to the planet from the solar flare that is heading towards the Earth. However, Danny reveals that they saw news that the government plans to use defoliating agents to strip the leaves from the trees to allow them to be burnt, which would affect their ability to stop the solar flare.

The School Trip

They return to the TARDIS where The Doctor uses his time machine to hack into the world's cellular network, and Maebh reads off a messages prepared by her and the rest of the children warning them not to harm the trees; she also asks for her sister Annabel to return. Outside the TARDIS Maebh is greeted by her mother, who has been searching for her. The Doctor offers the remaining group the chance to watch the solar flare's impact in space from the safety of the TARDIS but Danny turns down his offer saying that he will take the children back home. Clara joins him for part of the way during which Danny reveals he has figured Clara lied to him about travelling with The Doctor, but is willing to take her apology for it and continue their relationship. She and Danny then share a kiss, which the children laugh at because the rumours they heard have now been proven to be true.

Later, The Doctor and Clara watch the Earth from the open TARDIS doorway, as the solar flare harmlessly hits the Earth and the trees burn away. Elsewhere, Missy also watches the event on her tablet, calling the result a 'surprise.

On returning to Earth The Doctor and Clara watch, from in Clara's flat, the remains of the trees crumble away. Clara wonders what will be told about the trees in the future and The Doctor tells her that the entire event will have been forgotten, just as they did after the previous events, as it is human nature to forget about impossible things.

Maebh and her mother return to their home and as one of the bushes disperse they find Annebel waiting for them there, and the family has a tearful reunion.