Rigsby-who helped the Doctor and Clara against the Boneless a year ago and now is settled down with a wife and baby-now has a mysterious tattoo on his neck-one that’s moving; specifically, it’s counting down.
The Doctor and Clara trace the origin of the tattoo to a hidden street. A street full of aliens in human disguise-and run by the now 1300 year old “Me”-the former Ashildr. Turns out that Rigsby has been sentenced to death for the murder of an alien. When the Tatoo counts down to zero, Rigsby will die, killed by a mysterious raven.
However, it turns out the whole thing is in fact a ruse to lure the Doctor and have him hand over his confession dial, and the tattoo will be removed. However, unfortunately, Clara offered to take the deadly tattoo from Rigsby earlier, and while the tattoo could’ve been removed while still on Rigsby, it can not be removed while on Clara. Clara is going to die.
Clara faces the Raven, with it killing her as she stands.
The Doctor is very, very upset, and is very angry with Me. Giving her the confession dial, Me than transmats him to a mysterious castle, powered by gears, surrounded by a sea full of skulls, patrolled by a mysterious veiled monster.
The Doctor eventually realizes that the Veil-and further elements of the castle-can be unlocked by him confessing certain details-about the hybrid, the mysterious being Davros mentioned that could destroy Gallifrey. Revealing to the veil it was his fear of his knowledge of this that led to him fleeing Gallifrey, this confession opens the way to the exit. Unfortunately, the exit is blocked by a thick layer of diamond, that can only be unlocked by further confessing things.
The Doctor however won’t reveal anymore, and the veil burns him in front of the diamond wall. He’s also unable to regenerate.
Limping back to the transmat chamber, he dies, his body decaying to a skull, but before that he’s able to ‘reset’ himself into a loop, creating another him that goes through the same motions, but each time, is able to pick away at the diamond wall. Over billions of years, the Doctor punches through the diamond wall, each time only weakening it slowly. However, after billions of years the Doctor is able to break it, mentioning the Brothers Grimm. as he does so.
There’s this emperor, and he asks the shepherd’s boy how many seconds in eternity. And the shepherd’s boy says, ‘There’s this mountain of pure diamond. It takes an hour to climb it and an hour to go around it, and every hundred years a little bird comes and sharpens its beak on the diamond mountain. And when the entire mountain is chiseled away, the first second of eternity will have passed.’ You may think that’s a hell of a long time. Personally, I think that’s a hell of a bird.”
The Doctor escapes-turns out he was in his own confessional dial. And he winds up in a familiar site. He’s home-Gallifrey-outside the citadel of the Timelords-and he tells a local boy to tell the Time Lord’s he’s back and that the Hybrid-is me-as he puts his sonic sunglasses on.