After spending most of the fic making me(on a meta-level) think that he's the Master (Time Lord, but not the Doctor), he busts out with this. Context:Ace currently has a dolorovore (despair-eater) in her head.
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A memory of a conversation, some time after the event: "Professor? What did you use to hold off the haemovores? What do you have that much faith in?"
A pause; she thought for a moment he wasn't going to answer at all. He danced around so many other personal questions. But finally, he gave her his best I-have-a-secret smile, and said, "Names."
"What, like, names from Time Lord history? Rassilon, Omega . . ."
"No." His tone said, don't be silly. "Names of power. Important names."
He'd never elaborated. But this was it, this was the chant that the Doctor had used in nineteen forty-five to hold haemovores at bay, the words that held faith strong enough to make vampires cower–or dolorovores. There was only one person who'd know about it. There was only one person who'd believe in it, who'd be able to use it.
He'd been dropping clues all along. Deliberately, with the certainty that she wouldn't assemble them until he gave her the critical piece. Rolled r's. Apologies. Even mentioning his loathing of bus stations, to Britney, with a little offhand bit of reverse psychology to make sure she passed it on to Ace if she didn't take his offer of sleep. Oh, that was familiar, so very familiar. Ace would never be a chess master herself, but she knew enough to recognize a playing style.
She felt dizzy. Light-headed. Her mouth was hanging open. A rush of hope, all scrambled up with fear that it wasn't, because it was clearly impossible and rising joy, because impossible had never, ever applied, and what and how and am I dreaming? He was shouting the names now. Melodramatic, over-the-top–that was familiar too, the attitude of a performer. "Liz Shaw! Jo Grant! Sarah Jane Smith! Brigadier! Alastair! Gordon! Lethbridge-Stewart!"
Who Ace had met. And she'd heard of Jo Grant; never met her, but she'd seen one of her protests on some BBC news program and thought, interesting. She's got the Look.
Ordinary human names. Names of power. They were traveling companions.
The realization, the sudden totality of everything that meant, hit Ace like an ocean wave. The dolorovore's sticky tendrils weren't controlling her at all, now, but she was too shocked to move. "You," she whispered, as he bellowed names at the sky, "you–you're–you are fucking kidding me."
"Nyssa of Traken! Tegan Jovanka! Vislor Turlough!" His voice had risen even more. "Perpugilliam Brown! Melanie Bush!"
And then he stopped. And opened his eyes. And paused, for a long and unbearable moment, before stepping forward and tweaking her nose. "Ace," the Doctor whispered, "McShane."
Everything went white.
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(Explanation, a little later)
"Thought so." Ace turned back to Britney. "See, here's the thing. It worked. The creature jumped into my brain. The Doctor overwhelmed it with faith."
"No, I didn't," the Doctor said.
"What?"
"It wasn't feeding off me. I–did what I did–because I knew you'd recognize it. And from that, you'd know who I am. You fed it an instant of pure hope, and it burned up like a leaf in a supernova. All," he tapped her nose, "you."