The Doctor walks into his TARDIS. He remembers this, his home. It's as he left it-- no, not quite. The chalkboard, the jacket, the painting on the side. Not as it was. It is time to apply himself to the problem before him, and do it properly. Time to take stock. Time to nail down what remains in his memory and ensure he loses nothing more.
First thing to remember: the message on his chalkboard.
Run you clever boy
and be a doctor
This message was left by Clara, the Clara whom he could not remember, whose shape and existence and personality he must deduce from the holes left behind in him. She tried to erase herself from his TARDIS before she returned it to him, but the traces are still there, in the places she had to wipe. In this message, and in the red velvet coat she left for him. The coat he knows, somehow, she liked.
Be a Doctor. He puts the coat on, shoots his cuffs, adjusts the coat until everything is just so. The TARDIS feels the change, responds. She gives him a gift: a new sonic. It feels right in his hand, just heavy enough, in tune with his mind already. Blue lights ripple around it.
He is now a Doctor.
The second thing to remember is what being a Doctor means. It means figuring the puzzle out. It means defeating the puzzle. It means winning. "Tell me, teach," he hears himself say, "what does winning mean here?"
Why did he say that to himself? What does it mean? Who is the teacher?
The third thing to remember is that he is not an idiot. He has a perfectly functioning mind, if not a perfectly functioning memory, and he knows the sound of an early-model TARDIS when he hears one. He knows he was rescued and then left with his own TARDIS by a woman who understood more of his circumstances than a mere stranger would. Therefore, she is involved.
Commit her face to memory. Better: commit her face to paper. And do it now, before it slides away from him.
The Doctor finds his armchair, his two-thousand year diary, and the gnawed stub of a pencil. He opens the diary, finds the first blank page. He draws. Snub nose, dimples, large eyes, tears on her face as she listens to him. This woman, he is sure, is worth finding again.
He writes down what he can remember of her, of the diner, of the sounds, in his diary in pen, and in two languages just in case. Gallifreyan, his father-tongue, and English, his mother-tongue. Not that anyone but he knows this.
Next. The heart of the puzzle, of course, is his memory. He has been hit with a neural block device. A partially-malfunctioning neural block. The memories are there, but blocked away. Probably. Memory is holistic, holographic. One cannot wipe a specific memory completely without wiping the brain clean. The block will very likely yield to very careful work, if he can figure out exactly what was done to his mind.
No. Wrong verb. What he did to his own mind. He remembers choosing it. He remembers that, if nothing else. The words "I accept this" coming from his mouth.
Accept it? Why? He no longer understands that. He remembers the conviction that this was the only way, that neither one of them would ever give the other up voluntarily, that he was on the brink of destroying the universe to be with this Clara.
Clara. Clara. He needs a last name. UNIT, surely UNIT will help there.
He writes that down.
Fifth. He's seen that diner before. He's been in that diner, with Rory and Amy. He summoned them and himself to it, back when he'd been on a mission to avoid death for reasons too complicated and now-irrelevant to discuss. That meant the woman with the TARDIS, the woman who knew more than she ought about his story, was there as well. She was in his past, meddling. Meddling? Helping. Watching him. Because she remembers him. Because she is Clara.
She did not return to Gallifrey to die. She went somewhere else after she left him there in the Nevada desert, standing before his TARDIS. She's out there, alive.
"Gotcha," he says.
He closes his diary and stands before the chalkboard. He stares at the handwriting of the woman named Clara, the hole in his life. The woman who has a TARDIS like him. She's out there; she remembers him; she's waiting for him.
But this time, he'll need to be worthy of her.