The episode starts off with a young girl running through a forest towards the TARDIS. She knocks on the door and the Doctor pops his head out. She asks for his help to find her way out of the woods. At first the Doctor acts like he’s too busy to help her, but the young girl tells him she heard Clara’s voice telling her to go to the Doctor. He pops out his head again and notices that he is stuck in the middle of the woods. The Doctor allows her to come in and shows her around. He is busying trying to understand why the TARDIS is malfunctioning. It keeps repeating, “This is your final destination”. The Doctor gets frustrated as to why he is not in London, but Maebh Arden, the young girl, tells him that the machine is correct, they are in London. She shows the Doctor outside and Trafalgar Square is all covered in trees and vines. In fact all of London is covered in greenery.
Meanwhile Danny and Clara are gathering up their group of students from an overnight class sleepover at the museum. They try to exit the building only to find them standing in a forest of trees. It is later revealed that the whole planet is covered in trees. 
The Doctor phones Clara telling her what he has found and that Maebh is with him. Clara informs the Doctor that Maebh is fragile. She tells him that since her sister disappeared, and she’s been on medication to quiet the voices inside her head. Danny asks her if that was the Doctor. She tells him no, but Danny puts it together that Clara has been traveling with the Doctor, and lying to him all this time. She explains to Danny that right now they have more important things to worry about, and they will talk later about it.
The class, Danny and Clara make their way to Trafalgar Square where they are going to meet up with the Doctor and Meabh. While Maebh is waiting she is watching a government broadcast outlining their plans to battle this forest phenomenon and that everybody should stay in there homes.
The group finally gets to the TARDIS and Danny and the Doctor start to have strong words towards one another, Clara physically breaks them up. Just like the Doctor, he thinks out loud trying to understand the situation they are in. He then runs into the TARDIS with Danny right behind him. He notices Maebh’s school notebook on a seat. The Doctor grabs it and finds a picture of the sun reaching out towards the Earth. The Doctor realizes that Maebh has drawn events of what is going to happen. A solar flare just like the one on the planet Bank of Glabraxos is going to use the oxygen from the planetary trees to burn the Earth. The Doctor believes that she is the key to finding out who is behind this, but she is no where to be found. Clara and the Doctor go off to try a find her, leaving Danny behind. But soon after they are gone, a loud crash forces the group and Danny to follow them.
Breadcrumbs are left for the Doctor and Clara as they find Maebh’s phone on the ground. There several things scattered around the forest for Clara and the Doctor to find. They hear a scream in the distance to find her being chased by a pack of wolves from the zoo. However, the wolves are more frightened with something bigger, a tiger. Danny comes to the rescue and scares off the tiger. A small sweet reunion with Danny and Clara is interrupted by Maebh running away again waving her hands in the air. They follow her further into the forest until she stops and apologizes to the group. The Doctor tries to comfort her, telling that it is not her fault. She then tells them what has been going on with her. She says she sees thoughts in front of her in little fireflies. The Doctor discovers something and uses his screwdriver to reveal a swarm of golden intelligent creatures of light flying around her. It is revealed that they are an ancient race living in the greenery of the environment and will survive after humanity is gone. 
The Doctor asks them why did they send Maebh to him, but the entity denies sending her and doesn’t recognize him.
Realizing that this is not a monster he can fight and defeat, that it is ecological phenomenon that has been going on for thousands of years. This phenomenon actually occurred twice during the existence of humans; the Tunguska Event, and the Curuca Event. Seeing the Doctor’s expression, Clara realizes that there is no hope. She tells him to save the schoolchildren, Danny and her. But as she gets to the TARDIS, she does a complete reverse and tells the Doctor to leave without them and save another planet. That shocked him, and says that he can save at least the kids, or just her. Clara doesn’t want the kids to end up on a distant alien world or an asteroid, leaving them to wonder how they are going to survive. The Doctor asks her why, “Because I don’t want to be the last of my kind”. She says if this is how humanity is going to end then it should end for all of us; he reluctantly goes and flies off.
While sitting in the TARDIS, he has a theory about why the trees are there in the first place, and then it hits him. He rushes back to Earth and finds Clara, Danny and the kids. He tries to explain horribly to them that the trees are here to help against the solar flare. Like a shield, the trees oxygen will burn off the energy of the flare and leave the planet intact. He says that happened before in the past during the Tunguska and the Curuca Event; where a solar flare hit the Earth and tress protected the planet. Only thing is that the government is about to execute their plan to get rid of the trees. The Doctor tells them they need to broadcast a radio message to the world’s government to stop their plan in order for the planet to be saved. Maebh volunteers to send the message. It works, the flare came and the trees protected to earth.
Clara and the Doctor are in the TARDIS watching this, as well as the mysterious Missy. She is rather surprised that these events didn’t occur as she hoped it would be.
This episode didn’t have the same intense theme as the previous ones, but it is entertaining to watch the Doctor interact the children. Clara’s relationship Danny is still on shaky ground after the fact she has been lying to him. The Doctor looks rather concerned that Clara is becoming more like him. He is beginning to wonder who she really is. The next episode is the two part season finale and it is going to be a whopper. How will it end? We finally get a find out who Missy is and what is her connection with Clara.
