This episode kicks off with Maebh, a child in a red coat (Little Red Riding Hood, anyone?) running through a forest until she finds the TARDIS. What follows is perhaps the best scene in an excellent episode as she reads the TARDIS sign and proceeds to knock on the door and ask for help. The Doctor, as has become his way, is more than happy to leave her to her own devices until she asks "Are you The Doctor?" It's enough to get his attention and he kneels down to talk to her face-to-face, asking if she has an appointment.
Her innocence contrasts perfectly with his cynicism, but he's not as harsh with Maebh as we've seen him be with others -- he's a heck of a lot kinder to her than he was to Courtney in "Kill the Moon," never dismissing her entirely even if he's not quite paying attention to her.
Maebh is lost, which is no doubt in part because the forest they're in is actually the middle of London... which probably would have been more of a surprise reveal if it weren't in all of the trailers. Oh well. When the camera pans up to show a London skyline with trees everywhere, it's still pretty impressive.
But Maebh actually hasn't wandered far: she was part of a school trip Danny and Clara had escorted to he museum, and The Doctor calls Clara to have her removed. ("I'm a Time Lord, not a child minder!") When the Doctor, musing to himself, wonders how all of this happened, Maebh chimes in that the trees must be able to communicate in order to grow all at once like this. "So, what, you think that's how spring begins, with a group message on tree Facebook?"
Seeing Danny in action herding a group of reticent schoolkids is great -- especially since we've seen relatively little of Danny so far. It's clear from the start that the kids are important to him as he focuses on getting them to safety and, for the most part, ignores the madness of the forest-covered world around him. (Clara, meanwhile, marvels at the forest and has almost entirely forgotten she has kids to look after... remind you of anyone?)
Everyone meets up at the TARDIS, which gives The Doctor a chance to play grumpy -- okay, grumpier -- old man with a whole classroom's worth of kids trying to touch his TARDIS. But soon they realize that Maebh has gone missing again -- and The Doctor is very interested in finding her, since he's convinced she's communicating somehow with the trees. After all, she found him and knew his name... and seemed to know other things about the forest, though he didn't pay her much mind at the time.
The Doctor and Clara rush after her, leaving Danny and the kids in the TARDIS, which is the "safest place on the planet." So, of course, Danny leaves shortly afterwards, with all of the children in tow. It's one of those "let's create a dramatic moment by making the characters do something impossibly stupid" scenes, but the trope doesn't quite play out: instead, when Danny finds The Doctor, Clara, and Maebh being menaced by a tiger, he runs it off.... with a flashlight. (This is not something I'll be trying at home. Where I hope there are no tigers.)