Maebh, a young girl, is running through a forest when she comes across the TARDIS. She knocks and is answered by the Doctor, who after attempting to take her home, realizes that a forest has grown over London. Meanwhile in London Museum, Clara and Danny lead a group of kids out into the new forest after a museum sleepover. Clara immediately calls the Doctor, who reveals he is already aware of the forest and tells Clara to meet him in Trafalgar Square as he has found the runaway child from her group. Clara and Danny lead the children to the Doctor where upon arrival they attempt to figure out the truth behind the forest. Leading the children into the TARDIS, Danny notices a set of school books, and realizes Clara has been lying to him. The Doctor abruptly realizes Maebh is missing, and he and Clara set out to find her. Following a trail she has left, they find her, only to be cornered by wolves and a tiger which have escaped the zoo. As the Doctor desperately thinks of a plan, Danny arrives and drives the tiger away by shining a light in its eyes. The Doctor realizes Maebh is behind what's going on, and uses the sonic to get the trees to speak through her. They reveal a solar flare is heading towards Earth which will destroy it and wipe out all life. Heading back to the TARDIS, the Doctor offers Clara, Danny and the kids an escape route. Clara points out that she would never leave Danny, Danny would never leave the kids, and the kids would miss their parents. The Doctor reluctantly leaves, before realizing the trees have grown to protect Earth like a giant airbag and the solar flare wouldn't actually kill anyone. He hurriedly heads back to Earth, where, recruiting the kids, they send a message to the world to not destroy the trees. The solar flare passes by harmlessly and the excess trees disappear, their job done.