There’s a girl who volunteered to be frozen when her family fell on hard times.
There’s an old man whose heart has turned so hard within him it might as well be as frozen as the girl. In its way, perhaps it is.
The Doctor believes both can be thawed. It’s a bit tricky, but if that fellow Charles Dickens can change Scrooge…. Should be a piece of cake.
It’s not, there are false starts, but the Doctor’s never been a quitter. Change the past, just enough, and the future changes too. If he gets it just right, anything’s possible.