Once there was an ordinary girl you probably wouldn't notice if you passed her on the street. She was the kind of girl who worked in a shop and had a boyfriend she didn't really care about. She was smart and funny and a little bit tacky, with a tasteless mother. No wonder, sir, but certainly a maid. By coincidence and providence divine, she made a wonderful new friend, a man who was in league with time. He put his hand in hers and he said, "Run," and they were Rose and her Doctor, and they did. So far, and so fast, in fact, that they met themselves coming, and faced the Doctor's oldest enemies, and his final victims, and Rose found herself in league with time. It sounded like singing. But everything ends.
"I absorbed all the energy of the Time Vortex," the choking Doctor explains, "and no one's meant to do that. Time Lords have this little trick -- it's sort of a way of cheating death -- except..." Rose and the Doctor look at each other, light all around, the Doctor looking sick and not enough time to explain about the forty-year history of this show and how he always changes, and he always comes back. "It means I'm gong to change." And he explodes, golden light everywhere, and he turns into David Tennant. Who is very sexy, a little yippy, kind of ratlike, mostly nerdy, and quick, and funny, and angry as fuck.
You can't change your history, not even on Father's Day. But when it came down to it, the burden the Doctor carried, the trauma and pain of destruction of an entire race, was laid down, and he had the option of taking it back. He chose not to destroy them again. The Dalek were washed away in a wave of grace by a higher authority, but the Doctor's final guilt was gone, and with that his story was complete. If Nine was a god of grief and guilt, then Ten is what happens when that sin is removed, and he is a god of loneliness. Without even his guilt to keep him company. This is Her gift and this is Her price too, and since he's a new man, there's no chance he'll be able to explain it to Rose. Which fits a lovely parallel to the show itself, considering most of the audience hasn't been watching for forty years and it won't mean anything unless we get to mourn Nine and distrust Ten along with her.
The light drops away, and the regeneration finishes just as suddenly as it started. The Doctor tastes new teeth and smiles at Rose, hair all wild: "So where was I? Oh, that's right! Barcelona!" And Rose just stares, because this isn't her Doctor. He ignores her and her gobsmacked staring, the way she's hiding behind a TARDIS pillar, and starts turning switches and pressing buttons and being a good pilot. "6 PM, Tuesday, October...5006...On the way to Barcelona!" The Doctor faces Rose, grinning widely, and asks what he looks like, then holds up a palm: "No, no no, no no no no no no no. No. Don't tell me!" He counts legs, arms, hands -- "slight weakness in the dorsal tubercle," also known as Lister's, I've always loved that: "Lister's tubercle" -- and he's not bald, he's got sideburns -- "or really bad skin" -- and he's quite skinny. He's also got a mole, he feels, between his shoulder blades. Rose is breathing heavily; really scared, working up the courage to do something. "Go on then, tell me," he grins, hair ruffled and wild. "What do you think?" But what he means is: "Be collected. No more amazement; tell your piteous heart there's no harm done." But she can't hear it; she's too afraid to close the circuit.