Arrival

After witnessing the death of his companion Clara Oswald, in "Face the Raven", The Doctor is teleported into a glass chamber located in a structure resembling a large castle operated by clockwork gears. Still emotionally shaken by Clara's death The Doctor angrily speaks aloud to the entity that put him there, assuming that they are listening. The Doctor knows the distance that the teleporter is capable of sending him and he plans to wait until the stars come out so as to work out his location. Upon investigating the surrounding corridors, he finds a series of video screens scattered around the castle. The screens relay the vision of a cloaked creature, known as The Veil, which is slowly, but constantly, stalking The Doctor around the castle. The Veil pursues The Doctor to an apparent dead end. Admitting that he sees no way to escape death, The Doctor confesses that he is afraid to die, leading the creature to stop. The entire castle structure then rotates and reconfigures in response, opening an exit at the end of the corridor that The Doctor was trapped in, beyond which is a bedroom with an ancient, faded portrait of Clara. The Veil follows The Doctor into the room, forcing him to leap out of a window to escape. During his leap, The Doctor has an inner monologue, determining how he will survive the fall by imagining himself aboard the TARDIS answering questions written on a chalkboard from an otherwise-silent Clara. The Doctor lands into an ocean that is surrounding the castle. He sinks to seabed, finding that it is littered with thousands of skulls.

The Doctor returns to the castle, finding a room with a fireplace where exact copies of his clothes are drying. He swaps his wet clothes for the dried copies, leaving his old clothes drying in the same fashion as the copies he found. The Doctor quickly concludes the castle, elements within it, and The Veil are nightmares pulled from his memories specifically designed to frighten him for some purpose. Entering a courtyard of the castle, The Doctor finds a fresh grave and a spade, concluding that he is meant to dig the grave up. While digging, the castle's local sun sets and The Doctor notices that the constellations do not match any location within the range of the teleporter that brought him to the castle. After digging deep enough, The Doctor finds a stone with the message ‘I am in 12’. The Veil then suddenly reappears and corners The Doctor once again within the grave. The Doctor experiences another inner monologue within the TARDIS, where he remembers his earlier escape where The Veil stopped after he told the truth and realises that the creature is designed to extract confessions from him. The Doctor admits that he originally left Gallifrey because he was afraid rather than bored. The Veil once again stops, allowing The Doctor to escape.

The Doctor starts his search of the castle for Room 12, following the clue from the grave, however he is unable to find any door labelled as such. During his search, The Doctor finds that the rooms of the castle are constantly resetting themselves, as rooms that he revisits have any changes he has made reverted by the time he returns to them. Returning to the teleporter room where he first arrived, he finds a skull next to the word ‘bird’ written into some sand. Inspecting the stars again, The Doctor muses that 7,000 years must have passed since he was teleported, even though the teleporter did not transport him through time. The Doctor places the skull from the teleporter chamber on the castle ramparts, where it falls into the sea and joins the others on the seabed. The Doctor then resumes his search of the castle for Room 12 with The Veil still pursuing him. The Doctor finally tracks down Room 12 where he discovers another dead end. The Doctor realises that he will need to confess to The Veil again to change the castle's configuration and open the way beyond.

The Veil

The Doctor confronts The Veil again, confessing that he knows the identity and location of the Hybrid, a being prophesied by the Time Lords to be created from the mixing of two warrior races. This opens a route beyond Room 12, revealing a large wall made of Azbantium, a crystalline substance 400 times tougher than diamond. The Doctor realises that the message ‘bird’ found in the teleporter room was a reference to The Shepherd Boy by the Brothers Grimm, featuring a bird which slowly weathers away a mountain with its beak. With The Veil approaching, The Doctor again has an inner monologue in the TARDIS, where he admits that Clara's death and piecing together the scale and nature of the trap have damaged his will to continue. This time, Clara addresses him, telling him to get over her death and to keep going. The Doctor begins to punch the Azbantium wall, until The Veil touches his skin and burns him, disabling his regeneration process. The Doctor claws his way back to the teleporter chamber. Where he discovers that it has been reset to the state before he initially arrived and thus carries a copy of him as he was at the conclusion of "Face the Raven". The Doctor recognises that he has been in a recurring cycle for over 7,000 years, beginning with him emerging from the teleporter and ending with him sacrificing himself to provide the power needed to activate the teleporter and create a new copy of his younger self to restart the cycle. The old copy of The Doctor writes ‘bird’ on the floor before disintegrating, leaving only his skull behind.

The cycle continues for over four billion years, with The Doctor gradually wearing away part of the Azbantium wall each time before The Veil touches him. Eventually, he successfully breaks through the wall and sunlight pours in, causing The Veil to disintegrate before it can touch The Doctor. The Doctor steps out from a portal into a desert landscape overlooking the Citadel on Gallifrey. The opening closes behind him, and the item generating it drops to the ground. Picking it up, The Doctor discovers that he had been trapped inside his Confession Dial the entire time. A small boy then approaches. The Doctor tells the boy to go to the city and let the Time Lords know that he has arrived, having ‘taken the long way around’.

The Doctor Caught

In Nevada, The Doctor enters a diner and encounters a waitress physically identical to Clara. He begins to tell her a story about a girl he is looking for called Clara. Neither of them appears to recognise the other. In a flashback, to when he arrived on Gallifrey - having escaped from his Confession Dial - The Doctor approaches the old barn where he slept as a small boy. In the city Cloister Bells ring out alerting the Time Lord President, the High Council, and the Sisterhood of Karn of The Doctor’s arrival. This prompts the President to have The Doctor imprisoned, but others, including the Gallifreyan military, see him as a war hero, and instead they help to exile the President, who is revealed to be Rassilon, and the High Council. The Doctor learns that he had been trapped in the Confession Dial by Rassilon, to force him to reveal the identity of the Hybrid of the Gallifreyan prophecy that would conquer Gallifrey and stand in its ruins. The General and Ohila attempt to learn more about the Hybrid from The Doctor, but The Doctor asserts that they must ask Clara Oswald about it.

The Doctor has the Time Lords use an extraction chamber to retrieve Clara from her timeline just before the instant of her death, with her biological processes suspended in a time loop to stop her from dying, keeping her alive but leaving her without a pulse and unable to age. The General attempts to explain the situation to Clara, but The Doctor steals The General’s sidearm and, after confirming that he can still regenerate, shoots The General to cover his and Clara's escape. The Doctor takes a neuro block from the lab before the pair flee to the Cloisters containing The Matrix - the computer system that serves as a repository of the knowledge of dead Time Lords. While looking for the exit, and avoiding the Cloister Wraiths that protect it, The Doctor tells Clara about one Time Lord who managed to escape the Cloisters, leaving him mad. Clara recognises this was The Doctor himself, having learned of the prophecy of the Hybrid from the Wraiths that would lead him to leave Gallifrey in the stolen Type 40 TARDIS. The newly-regenerated female General and Ohila give chase and attempt to convince Clara to come with them and for The Doctor to tell them what he knows. Clara however, turns the tables, distracting them long enough for The Doctor to steal a new TARDIS from the workshop below the Cloisters, just as he did when he originally left Gallifrey.

The Doctor then takes Clara a long way from Gallifrey in an attempt to break the time loop so that she will regain her heartbeat. The Doctor hopes that he can escape having to return her to the moment of her death, despite potentially damaging time itself in the process. When it becomes apparent that Clara's timeline is not readjusting, The Doctor pilots the stolen TARDIS to the extreme end of the Universe, minutes before it is due to totally collapse. Having travelled only in time, not space, the TARDIS materialises in the ruins of Gallifrey. The Doctor answers a knock at the door and finds Ashildr waiting for him, having lived through the whole lifetime of the universe and becoming the last immortal being left. He accuses her of being the Hybrid, being a human modified with Mire technology. After The Doctor dismisses her idea that the Hybrid may instead be half-Time Lord and half-human, Ashildr presents her own theory on the Hybrid's identity: that The Doctor and Clara together are the Hybrid, since they are so alike, each pushing the other to potentially catastrophic actions. The Doctor then reveals his intention to erase Clara's memories of him, hoping that if she is left on Earth without any memory of him or her travels in the TARDIS that the Time Lords will not be able to find her and so will not return her to the point in time that she died.

The Doctor

Clara, who has been listening to them from inside the TARDIS, attempts to reverse the polarity of the neuro blocker using The Doctor's sonic sunglasses, so that it will backfire on The Doctor. When The Doctor and Ashildr enter the TARDIS, Clara reveals that she had watched them, and tells The Doctor that she is happy to accept her death but insists on retaining her memory. The Doctor doubts that Clara has successfully reversed the function of the neuro blocker, but concedes that he has gone too far to save Clara. The two agree to activate the neuro blocker together, not knowing which one of them will be affected. The device though affects The Doctor, who says his goodbyes to Clara before passing out. The Doctor then wakes in the Nevada desert with no idea how he had arrived or any idea who Clara is.

In the present, managing to piece together everything about Clara except what she looks like, The Doctor finishes telling his story to the waitress, who encourages him to keep going. While The Doctor is not looking the waitress goes to a back room, revealing Ashildr and the TARDIS console; the diner is in fact the new TARDIS that The Doctor stole, and the waitress is indeed Clara herself. The TARDIS departs, leaving The Doctor behind and revealing his own TARDIS, still covered with Rigsy's painted tribute to Clara.

Inside Clara’s TARDIS Ashildr reports that the chameleon circuit isn't working, so their TARDIS is stuck in the form of an American diner. Clara then declares her intent to return to Gallifrey to die, and so restore the timeline, as her death is a fixed point in time. But since she is now ageless, she decides to ‘take the long way around’, and sets off with Ashildr to travel the universe.

Meanwhile back inside The Doctor’s TARDIS, which comes back to life as he enters, The Doctor discovers a message from Clara on his blackboard: ‘Run you clever boy, and be a doctor’. The TARDIS then produces a new sonic screwdriver, and The Doctor closes the doors with a snap of his fingers, ready to continue his adventures. The Doctor then sets the TARDIS for a new destination with Rigsy's tribute to Clara flaking off the TARDIS as it dematerialises. The Doctor's TARDIS and Clara's TARDIS pass by each other mid-flight, before flying off in opposite directions on their own journeys through space and time.