The Doctor punches a wall for 4.5 billion years.
A teleporter in an old castle reconstitutes the Doctor’s body (he teleported here at the end of the last episode) and he wanders around trying to figure out where he is.
A creepy hooded creature, the Veil, chases him every now and then; when the Doctor gets cornered, he retreats into his mind to confer with a mental projection of Clara (because the actress is contractually obligated to be in every episode this season).
The Doctor finds a room with a thick wall made out of Azbantium, a material four hundred times stronger than diamond. The way out is on the other side, so he repeatedly punches the wall until the Veil shows up and burns his face.
Death of the Doctor
Dying and unable to regenerate, the Doctor stumbles into the room with the teleporter and uses the last of his energy to reconstitute a new version of himself, since his bio-pattern is stored in the teleporter’s computer. The dying Doctor fades away, leaving only his skull behind, and a new Doctor is created by the teleporter.
The new Doctor repeats the same cycle as before, but slowly makes progress at chipping away at the Azbantium wall. After 4.5 billion years, and 4.5 billion years’ worth of reconstituted Doctors, the latest Doctor finally makes it through and leaves the castle. Turns out he was in his Confession Dial the whole time, but now finds himself back on Gallifrey.
This was a very interesting idea for an experimental episode: it’s basically a one-man show starring the Doctor. Capaldi does a good job, and I respect that it digs deep into his psyche. The mood is effectively eerie and the Veil is suitably creepy; the scene of the Doctor finding a huge pile of skulls underwater is disturbing in of itself, but becomes even more so when we learn all the skulls belonged to his past selves.
But I had a hard time getting through this episode. I was bored through most of it, and kept looking at the clock waiting for it to end. The Doctor’s philosophical ramblings did nothing for me, and I don’t care enough about Clara to care about the Doctor getting over her death.
I know it’s a very popular episode that’s received countless praise, so don’t take my word for it. Maybe you’ll like it.