What's Past Is Prologue
"Who are you?" Rose asks, and the Doctor's smile falls: "I'm the Doctor?" But she shakes her head. She's been through a lot: "No. Where is he? Where's the Doctor? What have you done to him?" The Doctor points over his shoulder at the place where he regenerated, confused. Rose: "I saw him sort of explode, and then you replaced him, like a teleport, or a transmit, or a body swap or somefing." The Doctor stares, at a loss. She goes through her words: "You're not fooling me. I've seen all sorts of fings. Nanogenes, Gelf, Sliveen..." The Doctor woggles at Rose, just a tiny bit, and her jaw drops: "Oh my God, are you a Sliveen?" He tells her calmly that he's not a Slitheen, and she begins to shout: "Send him back. I'm warning you, send the Doctor back right now!" The Doctor pleads with Rose, leaning forward, as she heaves, explaining, "I was dying. To save my own life I changed my body. Every single cell, but...it's still me."
The Doctor steps up as Rose is whispering disbelief, saying, "Then how could I remember this? Very first word I ever said to you. Trapped in that cellar. Surrounded by shop window dummies, oh! Such a long time ago. I took your hand," and so he does, and the TARDIS sings as he touches her, "I said one word. Just one word, I said: run." Rose tears up and stares into the Doctor's face, unable to quite believe: "Doctor?" And he grins tenderly. "Hello."