In more than 30 years of watching Doctor Who, only Steven Moffat's has written a script that scared me. He has come damned close a second time with Listen.
The fourth episode of Series 8 started with the Doctor sitting on top of the Tardis as it floats in space. Then he carries on a monologue as he contemplates evolution and survival traits and arrives as hiding as the ultimate survival trait. Then ponders that if a species developed perfect hiding, how would we know they are there and how would we know if we are alone.
The Doctor quickly becomes obsessed with this idea and gets Clara's help in investigating. He plugs her into the Tardis telepathic circuits so he can find her dream of being grabbed by something under the bed. Unfortunately he keeps talking and he distracts her at a critical moment. Instead of going to her dream, they go to Danny Pink's dream (Clara's date for the evening... a date that did not go well).
Next stop, Danny's great-grandson, filled with hints that Danny and Clara are connected, but just shy of hints that Clara is his great-grandmother. Definitely hints that Danny will be on the Tardis. But for the Doctor, it is the creatures that knock on the airlock at night. He decides to let them in.
Clara quickly tried to use the telepathic circuits and they end up with a child crying in a barn. Clara quickly guesses the situation and what all has been going on.
Yet there was that figure on Danny's bed. Is the Doctor being paranoid or does he have something? We don't get the answer, but what we get is a creepy episode. Really creepy and we get to know the Doctor, Clara and Danny better. This episode was filled to the brim with Character Development and creepy.
Each episode of this series has been a very different type of story, but each has forwarded the characters and each has been excellent. Listen has an excellent resolution, even if the Doctor's question isn't answered. Moffat has done it again.