At the beginning of the episode we see a beautiful forest, which turns out to be in Central London. Trees have grown overnight and are covering the whole world.
Clara, Danny and some Coal Hill students have been at the London Natural History museum for a school trip when they discover what has happened. One of their pupils, Maebh Arden, has gone missing, but it turns out she has found the TARDIS and the Doctor in Trafalgar Square. Clara, Danny and the other pupils then go to meet the Doctor.
Clara tells the Doctor that Maebh’s sister Annabel went missing, and she has been acting strangely since then, hearing voices and drawing pictures of suns destroying trees. The Doctor believes that it may be connected to the forest, and that Maebh is tuned into a higher frequency than most people. He also says that a giant solar flare, much like the one seen in Time Heist, is due to hit Earth, and the consequences will be just as devastating.
The Doctor discovers that Maebh’s symptoms are coming from fairy-like creatures which appear to be like spirits of the trees. The creatures have been around almost as long as the Earth itself. The Doctor eventually works out that the trees are intending to protect Earth from the solar flare, and have grown in such huge numbers to act as a shield. This is far from the first time they have done it either.
The trees haven’t escaped the attention of the governments of the world, though none of them know about the upcoming solar flare. The government do not know about the solar flare, but are acting to get rid of the trees by burning them. However, the trees are able to fireproof themselves by controlling oxygen, putting the fires out. The governments of the world then plan to use chemicals to clear the leaves off the trees, thus making it easier to burn them down. Upon hearing this, the Doctor tells the group to all get in the TARDIS, and he asks Maebh to broadcast a message all over the world telling people to leave the trees alone. The solar flare happens, the trees are able to protect the Earth, once the flare has passed the trees which grew overnight disappear as they are no longer needed, and Maebh is reunited with her sister.
There is some interesting imagery in the episode, the sight of iconic London motifs such as red telephone and post boxes and black taxi cabs in the middle of a forest. Nelson’s Collom coming down and looking like plants have grown over it reminded me a bit of Planet Of The Apes and the famous scene of finding the Statue Of Liberty in ruins. It also contains a lot of literary references, notably when some wild animals are in the forest having escaped from the zoo. Maebh in a red coat being chased by a pack of wolves is even in the episode itself referenced to Red Riding Hood. The wolves are scared off by a tiger, and the episode title is a reference to The Tyger by William Blake.
But overall, this episode is very, very, very tedious. It’s not terrible, but the plot drags on. Perhaps it looks worse coming after three strong episodes, but it really does feel like filler.