"Instead of killing himself, he could rewrite his whole life and have a better one. He only had to use the Chameleon arch. And maybe finally be happy in a life where his past as killer wouldn't weight so much on his shoulders."
The last time Rose saw the Tardis the Doctor had left her with a piece of himself: a piece of himself that she's grown to love just as much as the original man. But when the Tardis appears in her world five years later, Rose can't help but be curious as to the reason why.