This is a very odd episode in so many ways. In one sense, it's a return to what made Doctor Who so good in the first place- the unknown destinations, the new monsters. But it suffered from some strange moments which slightly let it down overall.
The concept of the Doctor and his companion arriving in a base with some sort of monster on the loose isn't exactly a new idea and that's the whole point- it's a return to the style of Classic Who with its episode that were all so different from each other in terms of time, place and monsters. The excellent visuals and cinematography do a good job of giving this episode a lot of character and mystery. At once, we're introduced to the new monster (or ghost?), which is instantly memorable. The ghosts are scary, mysterious and horrifying, and the most intriguing element of this episode. It's great to see Doctor Who return to its core: the Doctor and his companion in a different location in a different time with some sort of different enemy that they have to work out how to defeat or escape.
That's all well and good, but what kind of let down the episode for me was first of all the dialogue. It was really strange and awkward in places, and it was partly due to the odd dialogue that the episode struggled to establish tension. It's hard to care much for the crew because no personality really comes out of them at all. Peter Capaldi's and acting was so-so, Clara was particularly, exaggeratedly excited at the start of the episode and the Doctor was particularly, exaggeratedly serious. Later on, in an out of place and awkwardly performed scene in the TARDIS, the Doctor starts asking Clara if she's okay? That he thinks she should be having a normal life? It's completely out of the blue and is an example of how the episode really got quite strange and clunky at points.
Thankfully, the story maintains intrigue very well as the Doctor slowly unravels the mystery. At the same time as I think the cliffhanger was silly because we all know it won't actually happen, the ways the story unravels makes me look forward to the next episode, which there's a big chance will impress just as well if not more.