"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."
Rose stumbles backward in the impact; the Doctor takes off around the console: "And we never stopped, did we? All across the universe. Running, running, running...One time we had to hop, do you remember? Hopping for our lives?" He hops madly, grinning, trying to get her back on his side. "Yeah? All that hopping? Remember hopping for your life? Yeah? Hop?" He stops hopping. "...No?" Rose asks if he can change back, and he asks if that's what she wants, and she says it is, and he is crestfallen. His voice is blank but his face is sad. He can't change back. "Do you want to leave?" She's shocked: "Do you want me to leave?" He rushes into the gap, scared and lonely already: "No! But...your choice, if you want to go home." He calls her bluff -- note how she's not actually involved in this part, so much, except in her silence -- and cancels Barcelona: "Change to London, the Powell Estate...Let's say the 24th of December. Consider it a Christmas present," he says, giving her the eye. Begging her to stop him. "There." He rocks back on his heels, arms across his body, defensively, saying, "I'll show you." The TARDIS shudders in the red, back toward home. "Up to you. Back to your mum...It's all waiting, fish and chips, sausage and mash, beans on toast -- No, Christmas! Turkey! Although having met your mother, nut loaf would be more appropriate." (Ha! But also, how can you say it's lazy writing to be grossed out by British food, when what the show just said is "It's not a joke, it's just true.") Rose hides her smile at this, and the Doctor teases her knowingly. She eventually resists, pushing back a bit too hard, and he rolls his eyes, exasperated: "Oh come on, all I did was change! I didn't..." The TARDIS jerks and he gags, suddenly. What could the end of that sentence have possibly been? Rose is confused, and the Doctor almost drops, retching and jerking. "The change is going a bit wrong and all," he explains, dropping to his knees.