"Night Terrors" is ninth episode of the sixth series of Doctor Who. It stars Matt Smith as The Eleventh Doctor, Karen Gillam as Amy Pond and Arthur Darvill as Rory Williams.
"Today we're answering a cry for help from the scariest place in the universe. A child's bedroom."
We take a brief break from the convoluted story arc of the season for what is (at first glance) a monster of the week episode. While traveling through deep space, The Doctor receives a message on his psychic paper, begging for help against monsters. The Doctor and his companions find that it originates in contemporary Britain, where a small child is having scarier and scarier nightmares. When The Doctor goes to investigate, he finds out that there is something more to this than just a frightened child: his sonic screwdriver has found power readings off the chart. Meanwhile, Amy and Rory have gotten separated from him, and find themselves in a strange building, being pursued by malevolent dolls. And, as is always the case, the mystery and its solution are a bit deeper than it would first seem.
While many Doctor Who episodes dabble in horror, this is one of the episodes that plays just like a horror movie, all the way through. And like much of the horror in Doctor Who, it is based on psychology and things left unseen, rather than on anything gory or explicit. Although some of the material has been done before (possessed dolls are somewhat of a cliché by now), it is all done very well, and the episode is frightening.
I also think this episode fits into the arc more than might be apparent at first. In "A Good Man Goes To War", River Song says that The Doctor will fall further than he has ever been before. In this episode, and in the next two, there are three mysteries presented, and each time, The Doctor fails to understand the mystery. And, although I might be shoehorning things too far, the mysteries involve, respectively, identity, time and space. The Doctor, as a Time Lord, should be above such things, but he is instead tricked and manipulated by the world of appearances, by the world of reflections. So this episode is a strong episode on its own that fits into a deeper meaning of the story and character arc.