This episode is simply described as a mess. Lots of positive qualities to it and even some very moving moments but it feels like a very enthusiastic kid telling you a story that wanders off in ways that clearly make sense to the story teller but not to the audience.
I won’t detail the plot because it’s not easy to do so in broad brushstrokes. There are three quite different elements to the story though.
The first is some genuinely creepy monster horror that mixes big scary monsters with a creepy guy manifesting near your bed as you sleep. Doctor Who has been a source of childhood nightmares for multiple generations of British children and this episode will fuel several. Even the weaker parts (detachable fingers in your ears) pick up on the horrific absurdity of nightmares.
The second is a bit of pseudo-mythology about god like beings (with shout outs to other uses of this idea in past Doctor Who) and their schemes. This is the least successful aspect of the story but I did like the break into an animated back-story/myth.
The third is Doctor Who does mental health which is less bad than it could have been, bordering on good, but maybe a bit beyond what the show is capable of dealing with. We have several characters dealing with their psychological problems (including in 14th Century Syria) which is connected with events but while there is a connection between mental illness and nightmare events, the episode narrowly avoids the lazy trope of mentally-ill psychic is the nexus of horrific events.
At long last this episode does make a solid attempt at giving each of the companions their own stories. Yaz, Graham and Ryan each have their own encounters with the spooky phenomenon and later must each confront their own fears. We even get a bit of an origin story for Yaz (i.e. why she became a police officer).
So not terrible and in so far as it really didn’t work, it wasn’t through a lack of ambition. The story does require the Doctor to be a bit obviously thoughtless when encountering a giant cosmic prison and the bad guys are so camp that they can’t live up to the spooky build up. Yet the episode has its own corny charm that’s very Doctor Who and once again I had no idea where things were heading for a fair degree of the episode.