This episode starts with a crew of an underwater base in Scotland discovering an alien craft. As long time Doctor Who fans know, that is never good. This leads to the death of a crew member, who appears to become a ghost. Another trope of Doctor Who, anything that seems supernatural, never is. The Doctor and Clara arrive three days after the first incident. It seems as though the apparitions are attempting to kill everyone in order to create more ghosts.
This is another two-part arch. It looks like this is what the program is doing this series. It also allows the BBC to put out smaller collections o DVD and Blu-ray. Many concepts used in this episode border between the scientific and spiritual. Doctor Who has done that many time, it ventures into the realm of the metaphysical.
This episode kept the suspense up. They played into the ghost story aspect with the isolation of an under water base. They hinted at a gothic theme with one of the ghosts being dressed in what appeared to be Victorian era clothing. Once again we get an episode that is all setup. The cliffhanger was momentous; it left us, the audience, wondering if the Doctor will get out of this in one piece. We will see in the next episode. I give this episode a B.