"This is like a disk, a cyberdisk, but why would the Cybermen need something so simple? They've got to be wireless! Unless... They're in the wrong century, they haven't got much power, they need plain old basic infostamps to update themselves..." John finally notices the Doctor, his Doctor, staring into space, losing track. He puts on his glasses; Smith doctors the Doctor. "What is it?"
"I've seen one of these before," the Doctor says in a harsh, sad voice; the Doctor fights past one lost memory after another: Cybermen attacking, he was holding the device, the infostamp, when they... "The night I lost my mind. The night I regenerated." His face, changing before your eyes. "The Cybermen, they made me change. My mind, my face, my whole self." He puts his hand on John Smith's face, remembering him briefly. Feeling closer to him, for a moment, than a brother. Doctor-Doctor, inside. "And you were there. Who are you?" John Smith looks back at him, realizing he's been playing them both false this whole time. Where there were two Doctors, they thought, there were really none.
The Doctor looks at this man, this nameless man who lives in an improvised, jury-rigged approximation of his broken life, who does with it more good and more joyfully than he can remember feeling capable of. Rose, the Master, Donna. His daughters. Martha. "A friend," he says lovingly, giving the man all the strength he can master. "I swear." The man, the Next Doctor, begins to weep. "Then I beg you, John. Help me." Two words the Doctor never refuses. He watches his Companion, this strange man, go down again, into the shadows, and he loves him.
"But it's not a conversation for a dead man's house. It'll make more sense if we go back to the TARDIS... Um, your TARDIS." The man doesn't respond. It's too close, the pain is everywhere, implicit in the dead man's house, in the house of the man who died. Or was born. The Doctor rushes around his friend, sonicking everything: "If this room's got infostamps, then maybe, just maybe, it's got something that needs infostamping..." he opens a door, and sees a Cyberman staring down. And so begins the chase, which has the benefit of getting the stranger moving again. The Doctor shoves him up the stairs and grabs first an umbrella -- he flaps it open and closed again, which does nothing, all the umbrellas in London couldn't stop this thing -- and then a cutlass, which is suitably ridiculous, and the entire time he's trying desperately to help them.