"Listen, whatever you're doing stuck in 1851, I can help! I'm the only person in the world who can help you! Listen to me! I'm the Doctor. You need me. Check your memory banks, my name's The Doctor. Leave this man alone, the Doctor is me! The Doctor, remember? I'm The Doctor! You need me alive! You need The Doctor, and that's me!" The other man runs, up the stairs, and comes to rest as the Cybermen continue to advance, chasing after them both, screaming Delete! Delete!
He remembers holding the infostamp, holding the infostamp. He remembers it flashing, as it flashes, and then he zaps the Cybermen with it, until their heads explode. Information overload, literally; the Doctor salivates on him: "Infostamp with a cyclo-Steinham core. You ripped open the core and broke the safety! Only the Doctor would think of that!" The last next doctor nods. "I did that last time," he says, remembering their attack in a dead man's house. The Doctor produces a stethoscope out of nowhere, promising he'll be okay, checking both sides of his chest of course. "You told them you were The Doctor. Why did you do that?" And the Doctor's answer is true, and false, and more of both than anything he'll ever say: "Oh, I was just protecting you."
The man's face twists, because he can sense the darkness behind it; the Doctor knows something about the Doctor, something ugly and lonely, a death and memories lost, and the blood of children, and the silence of women, and the engine that takes us where we need to go. "You're trying to take away the only thing I've got, like they did." Like they did, like they always have, like they will continue to do. "They stole something, something so precious! But I can't remember. What happened to me? What did they do?"
The whole point of coming back here, after all the past iffiness with Victoria, was for this moment. The moment John Smith could look this strange, nameless, history-less man in the eyes and see all that strength, all that power and wisdom, the way he laughs at death, and then see the weakness and the fear and the deep unknowable sadness too. So that he could look at this man, be his Companion and realize that no matter what, he has to protect him, save him, help him solve the mystery of himself. The Doctor nearly cradles him now; he's lost so much. "We'll find out. You and me, together." This is what she felt. This is what they all felt. Now he knows. It only makes his heart ache more.