"So these are my replacements, a dandy and a clown." - The First Doctor.
The Doctor doesn't warn them that it's going to happen. What would be the point? They wouldn't remember it afterwards anyway, and would just end up laughing at him.
But he himself can at least remember who it's going to be, and ever since he saw Zoe, he'd been wondering. Better to keep it to himself, he thinks. No point in startling them, lovely girl, and Jamie of course. So he strikes a note in his recorder. It always makes him feel better. Clears the mind. But he's delighted to see Susan again, he gives her a hug, why wouldn't he? Of course he knows now that it probably was meeting all of the others that made him leave her there in 2164 in the first place but she's here and now and oh, how he has missed her! And besides, she seems quite happy with the answer she gets, that she's off having adventures of her own. And she is. A great adventure. One he seems incapable of having.
So he argues again with the dandy one who comes to say hello to Jamie and Zoe but ignores him completely. He remembers seeing it the first time round and being exasperated at the two of them and yes — here is the eye rolling from the old man with no sense of humour — or musical taste. He strikes another note in the recorded but just because he noticed the dandy one, who knows the one with the disturbing eyes is there now, sneaking a look. Of course, the Doctor has had enough time to ponder whether this would be the end, after all, all of his regenerations are here in order but none after him and he might as well die once they all leave. But it's quite the gloomy thought and the Doctor would rather concentrate on other things. Absent mindedly, he takes a jelly baby from the Mad Hatter, the one with the smile of a Cheshire cat.
Don't wander off, he calls after Jamie and Zoe, a bit afraid of whatever gossip that might be going around in these circles, and instead instructs them to help him with the cords. They are soon assisted by another human girl — the lovely blonde one called Rose, and the Doctor quite likes her, even though she's quite confused, bless her. But she's doing her best to help. Of course, helping in this case means tripping all over, unplugging a cord and being shouted at by the rude yellow one, but he just pats the girl and gets her to help him instead, and then in no time there's the explosion from the direction of the dandy one and the one with the ears ends up joining them from out of his Tardis — he's not quite sure how much time he was in there to convince him, but by his reckoning and memory can't be too much, and he wonders what he said — would say — to himself. But now the five of them are working and Zoe, Jamie and Rose are all exchanging anecdotes — about him no less! Oh, the cheekiness of these humans. But yeah, both he and the one with the big ears can't help but smile, too.
It's over all too soon for the Doctor's liking — he doesn't know why he dreaded it so much in the first place. He gives another hug to Susan even though the old grumpy one doesn't approve — oh, does he ever! — and the two of them are off, and the Doctor smiles fondly as he remembers the meeting they are both about to have. How he would have liked to meet Ian and Barbara once again! But that was then and this is now and he knows it's their time to go. Jamie and Zoe won't remember a thing of what's happened, of course. Their human minds just aren't built for it. But he will, and that's enough. Rose gives him a hug and he's quite surprised, and then goes back and holds his hand, the one with the big ears, and if the Doctor catches disturbing eyes looking at them in a weird way he dismisses it immediately and off they go, he ushers Jamie and Zoe back into the Tardis, back into their own time line, there are so many more adventures to be had.
They land in World War One, or so they think at first, and the Doctor is quite sad when he realises he would never see Zoe and Jamie again save these little moments outside of time and space. Perhaps he shouldn't have been so rude to the dandy one who was only trying to say hello. He does his best not to think of the dandy one anyway — why can't he ask the great Council and those meddlesome Time Lords for different regenerations? — and since he can't decide for himself, they do it for him and he has quite a nasty feeling before he passes out.