There are many worthy aspects of the 10th episode of the series. The interaction between Clara and Danny Pink works well and Danny continues to show appropriate concern for the pupils in their care (a real-world care that Clara sometimes fails to share). He even gets to save the Doctor and Clara from an escaped zoo animal. Most importantly, the basic scenario of the world suddenly turning into a forest overnight is truly wonderful and it's easy to see why this script got commissioned. Unfortunately, though, the initial promise is never fulfilled and the episode eventually limps to a cop-out ending featuring glowing alien lights that have protected planet Earth forever... that nobody has never heard of. Despite multiple prior threats that were just as serious, these lights save the world from solar activity with a blanket of oxygen. Seriously. Oh, and then the trees vanish just because it's convenient. The scientifically inept ending merely confirms what audiences could see coming - the writer had a good central idea for the story but couldn't find a satisfying way of ending it. This serious flaw reduces the episode to merely being good and in a series this great, that grade puts it firmly in last place.