Maebh hears voices -- which she's usually on medication for, but with the whole "forest appearing overnight" situation, she hasn't taken he pills and is acting a bit erratic, swatting at the air. It turns out that she's communicating with something called The Here -- yes, that's another definite article character, a conceit that's getting a lot of play this season. This group of trees claims to have been here since the beginning, as "the life that prevails." The Doctor was hoping that they caused the solar flare that's rushing to Earth and could stop it... but they didn't and offer no answers.
The Doctor is out of solutions. He's not sure how to stop this, and that's where things get interesting with Clara. In a very Doctorish move, she says they have to save what they can and escorts everyone back to the TARDIS... but when they arrive, she insists The Doctor leave on his own. The kids would never stop missing their parents, Danny would never agree to leave without them, and Clara won't go alone to become the last of her kind. This is a bait and switch we've seen The Doctor pull on his companions plenty of times, and it's interesting to see the tables finally turned.
However, it's more interesting to see Clara not only tricking The Doctor into leaving but also being willing to allow their entire group to die without consulting Danny or anyone else as to their fate. Would being dead really be worse than being alive and missing your parents? It seems like a very harsh move for Clara to make, but it fits in with how we've seen her learning from The Doctor this season.
But because we have two more episodes left, we know this can't be the end, and it's not: the trees, it turns out, have arrived to protect the earth from the solar flare, providing a cushion of oxygen around the planet that will prevent it from burning to a crisp. (Don't think about this too hard and you'll be alright.)
The other thing worth noting this episode is the dynamic between Clara and Danny. Clara's been telling him increasingly outrageous lies to cover that she's been traveling with The Doctor and after last episode's phone call during a near-death-experience, it's frankly shocking that Danny hasn't realized what's going on. This episode, of course, makes it pretty obvious... and Clara's oddly more concerned about the fact that Danny might be on to her than the fact that a solar flare is going to kill them all.
Considering Danny had picked up on something wrong with Clara before she told him about The Doctor, it seems as though he'd catch her having the same odd behavior -- and really dreadful, dreadful lies. Instead, we get to see his sad face when he realizes she's been lying to him... and he entreats her to be honest with him moving forward.
Because, right, that worked last time it around.
Regardless, the end of season 8 is fast approaching... and the trailer suggests we're in for a doozy. Stay tuned!
Bottom Line: Though this episode does little to advance the plot, it's an entertaining side story with some truly great character moments.
Recommendation: So long as you go in not expecting big plot development, you'll probably enjoy this one.