This is the second half of the second two part story arch for this series of Doctor Who. It open with the Doctor explaining a causal loop or boot strap paradox with using Beethoven as the subject of his thought experiment. This leads to a guitar rendition of the opening notes of Beethoven’s Fifth and a electric guitar ladened Doctor Who theme.The Doctor goes back in time to before the town near the underwater base was flooded. We discover the spaceship was a hearse carrying an alien warlord. Clara and the Doctor talk about the new ghost in the future with each other using the TARDIS’ ability to connect temporally to cell phones. It is a trick first introduced by the 9th Doctor in Series 1.
When they discover that the new ghost is reciting a new list, a list of the order of deaths, the Doctor does not like it that Clara is next. He tries to travel to the present, but the TARDIS does not allow him to leave the past. He resolves the conundrum he finds himself in. He however finds himself in a causal loop also known as a boot strap paradox. He wonders who really came up with the solution to their dilemma.
The Doctor and Clara are put to task about their willingness to put others at risk. It presents the idea that maybe some of the Doctor is rubbing off on Clara. Doctor Who also as a program examines and plays with all the time travel tropes and paradoxes. Overall it is a great episode. I give it a B.