Plot
The Twelfth Doctor is teleported into a glass chamber within an empty castle in the sea. He is pursued by a cloaked veiled figure, [N 1] which evokes the Doctor's childhood fears. When the figure corners the Doctor, he admits he is afraid to die. The figure halts and the castle reconfigures itself. The Doctor jumps out of a window into the sea, finding many skulls under the water. The Doctor concludes that the castle is a torture chamber. He takes advantage of the figure's slow reaction time to explore the castle, finding several strange remnants of his predecessors, such as dry replacement clothes, a skull connected to the transportation chamber, and the word "bird" written in dust. The Doctor investigates clues left in the castle, directing him to find Room 12. He delays the figure by alleging knowledge of the Hybrid; the movement of the reconfiguring castle causes the skull to fall into the sea.
Inside Room 12, the Doctor discovers a wall of Azbantium, a mineral harder than diamond, behind which he theorises the TARDIS to be. He realises that "bird" refers to "The Shepherd Boy", a fairytale by the Brothers Grimm in which a shepherd's boy says to an Emperor that a second of eternity will have passed when a bird chisels a diamond mountain down to nothing with its beak. The Doctor temporarily despairs following an epiphany: the prison was made solely for him, and thus the skulls were his own and he has been in the castle for 7000 years. Revitalised by a vision of his dead companion, Clara, the Doctor punches the wall while reciting the fable. The figure mortally injures the Doctor, disabling his regeneration. He crawls back to the teleportation chamber, and burns his body as a catalyst to restart the teleport, aware that due to the resetting rooms, a copy of himself is inside. He then falls to the ground, writes "bird" in the dust, and disintegrates, reduced to the skull which the Doctor had seen earlier. A new Doctor appears, starting the cycle anew.
After many more cycles, [N 2] the Doctor finally breaks through the wall. He finds himself outside the Capitol on Gallifrey; the castle was in his confession dial. [N 3] The Doctor tells a young boy to tell the Time Lords that he is on his way and that he knows what they did. He then proclaims the Hybrid prophesied by the Time Lords is "Me". [1]
Continuity
As he walks down the corridor, the Doctor says to his unseen adversary "the Doctor will see you now". The Eleventh Doctor shouts this same line to the Atraxi in "The Eleventh Hour" (2010). [2]
The Doctor tells himself "Assume you're going to survive. Always assume that." This is what Clara says of the Doctor in "The Witch's Familiar": "he always assumes he's going to win. He always knows there's a way to survive". [3]
The Doctor confesses that he ran from Gallifrey because he was scared, and that the pretense of being bored was a lie. Originally, in The War Games (1969), the Second Doctor admitted to his companions that "Well, I was bored". [3]
Once he arrives on Gallifrey, the Doctor tells the young boy to announce that he "came the long way around", finally finishing the objective set by the Eleventh Doctor in "The Day of the Doctor" (2013) saying that he was going "home, the long way around". [3] [4]
Throughout the episode, the Doctor mentions the "hybrid", echoing a prophecy related by Davros in "The Magician's Apprentice" / "The Witch's Familiar" – two great warrior races, speculated to be the Time Lords and the Daleks, would merge to become a "hybrid". "Hell Bent" clarifies that the Doctor's declaration that the hybrid is "Me" expresses his belief that it is Ashildr, who became a human-Mire hybrid in "The Girl Who Died" and later adopted the name Me.
