When the Doctor decided to become a human, I didn't really think about what that would actually mean. I suppose I thought it'd be like Wife Swap or something. I mean, I'm trying to imagine swapping my life with Leo. I love being Aunty Marth but the idea of giving up what I have, to be all settled down with a kid... yikes! But for the Doctor it was so much more than that.

He's lived for hundreds of years. I mean, that's big. That must change the way you look at anything. I've been thinking back to what he said to Lazarus – about how it's not how long you live but what you do with your life that matters. And I'm sure he meant it but it's not true, is it? The last couple of months I've spent with him have been the biggest thing in my life. But for him it must be the equivalent of like a conversation I once had with some stranger on the Tube. He's probably always going to be the most important person I'll ever meet but I must just be a small distraction to him.

The thing is, when he was a human, I told him something. I'm not ready to write it down here although I reckon you can probably guess what it was. The problem was that he fell in love. With someone else. She was called Joan and she was nice and everything and she loved him back. She'll never forget him but he'll forget her. The human John Smith gave up everything so that he could save the day and I think he did it more for her than for me or anyone else. He loved her in a way the Doctor can never do. He'll move on and she and everyone else we met will be forgotten. People like Jenny. Jenny was great. We were mates and then the Family came and took her because they were looking for the Doctor. And Tim. Such a little sweetheart. Going to be a real heartbreaker when he grows up. If he grows up, that is. Because that's the thing, isn't it? We've left Joan and Tim and everyone else back in 1913 and for them, the world is going to change and they're all going to be at war and some of them will die. And they'll be forgotten. In the same way that Joan was a tiny part of the Doctor's life, so they're all just a tiny part in history.

But I won't forget them and I know Joan will never forget John Smith.

And I just hope the Doctor will never forget me.

I was just about to send this off when the Doctor said he wanted to show me something. We left the TARDIS and we were back in my time. For a minute I was worried he was bringing me home again but then he told me to look where we were. We were in the same village but, like I say, so many years later. And there was some kind of service going on next to a war memorial. We watched from a distance and then the Doctor whispered in my ear that the old bloke in the wheelchair was Tim. He'd survived both wars and God knows what else! And there he was. In front of me. So much older but alive. And I realised that the memorial, like the one back on in the Eye of Orion, meant that we didn't forget the people we left behind.

And I was wrong. What I said earlier about the Doctor. That was wrong. Simply by bringing me here, he was showing me that he didn't forget.