The Doctor and I stand together, staring into a mirror. For some reason, the TARDIS had taken us to a warehouse in the middle of New York. When we looked around, we found it was empty – except for the mirror on a stand against one of the walls. By all appearances, it had looked like your typical Ikea mirror, apart from the fact our reflections aren't showing.

"Very Harry Potter," I say quietly. "Though I did think my heart's desire would be something more than an empty warehouse."

The Doctor reaches out with her hand and to touch the glass but it ripples around her fingertips, a golden glow parting to reveal a darkness behind.

"I think it's a portal," she breathes, pulling her hand back.

"Where to?"

"I dunno," she says, grinning back at me. "But I think it's time you came through the looking glass, Alice!"

I roll my eyes. "Nine weeks it's taken you to make an Alice in Wonderland reference. I think the universe got impatient."

The Doctor chuckles. "Wanna have a look?"

"Of course!"

"Could be dangerous."

"Or it could just be Tweedledum and Tweedledee," I shrug. "When has our adventures ever not been dangerous? It's part of the fun!"

The Doctor grins. "A girl after my own heart! Come on then!"

She goes in through first, and I follow after her a few seconds later. For a moment as I pass through, it feels as though all the air is being sucked out of me, and I start to feel a choking sensation. Then, in a heartbeat, I can breathe.

I look around at the dark cave around us. "I'm gonna go out on a limb and say we're not on Earth any more."

"I think you might be right," the Doctor says, also looking around. As we try to work out where to go, a childish laughter echoes through the cave. It sounds like a little girl. "Hello? Is anyone here?"

A flash of red darts out from behind one of the crevices, then disappears as quick as it appeared. The Doctor's face grows suddenly serious, and she stiffens. Something's wrong.

"Shall we follow it?" I ask, starting towards the direction it came from, and after a second the Doctor nods and I follow behind her as we walk through the cave.

A short way in, we hear the laughter again. I look around to see where it came from and see a red balloon tied to a rock nearby. The Doctor sees it too, and a flash of recognition crosses her face before she looks away and walks on. She knows something.

"Doctor, what is it?" I ask, jogging to keep up as she quickens her pace. "What's in here with us?" Before she can answer, we turn a corner and see another mirror ahead of us and the Doctor runs towards it. I follow after her and pass through the mirror a second later, but then frown as I look around. "We're back in the warehouse."

"Not quite," the Doctor says, still sounding a bit on edge. I look back at her, and my eyes widen.

"Your parting – it's on the other side now! We're on the other side of the mirror!"

"Yes you are, you clever girl." I freeze as I hear the familiar voice, then slowly turn around.

"Oh that's not fair!" I cry out, looking over at my Nan. "You can't be here."

"I think you were spot on with your Harry Potter theory," the Doctor says quietly.

"Where are we?" I ask, my voice hoarse and my eyes still fixed on my smiling Nan.

"Still working on it," she replies.

"You're in the solitract universe," another voice says, and a young blonde woman walks out from one of the side-doors and strolls into the centre of the warehouse, before grinning over to the Doctor. "Wotcha!"

"Rose," the Doctor breathes. "No, that's not right." Her head snaps up. "Hang on, did you say solitract?"

"What's a solitract?" I ask.

"Think of Harry Potter," the Doctor explains. "He looks into the mirror of Erised and sees his parents – the thing he wants most in the whole universe. So he stays. Right at the beginning, the solitract was part of our universe, but it was like an extra piece in a jigsaw so it got thrown away, but it wants to come back – it wants to be part of our universe. But the solitract can't move through the mirror to join us, so parts of our universe come through to join this one and stay because what they see is what they want most in all the universe. Understand?"

"Think so," I nod. "So she's not my real Nan?"

"'Fraid not. We should go."

"That's not quite true," the girl – Rose – says, walking over. "We're real – just as real as you or the Doctor. Same memories, same thoughts, same everything." She says the last bit looking pointedly at the Doctor. "You could have a life, here, with us. It could be like it was always meant to be."

"But it can't," the Doctor says firmly. "You know it can't, Rose."

Rose smiles sadly. "Yeah, I know. I've seen you trying to stay still." She laughs. "It never works out well."

"Hang on, if the solitract wants us to stay, why are you trying to get rid of us?" I ask.

"Because she's Rose Tyler," the Doctor says in awe. "Brilliant in all universes."

"That's me!" Rose grins, then looks to me. "Started out as a shop girl, same age as you. But the Doctor taught be how to live a better life, as I'm sure she has with you too. I've seen a world without the Doctor, and I have no intention of seeing it again."

"Thank you, Rose," the Doctor says, walking over to her and the two embrace for a long moment, before Rose Tyler fades out of existence. The Doctor wipes a silent tear away before turning and walking back towards the mirror. "I'll give you a minute to say goodbye to your Nan, but then I'm going to have to close the portal. We can't risk anyone else wandering in here.

"I know," I nod, and the Doctor gives me a small smile before stepping through the mirror. I turn back to my Nan, and my heart tightens as I see her crying silently. "I have to go."

She grabs hold of my hand. "You can't leave me here on my own. Please – I remember everything. I remember the trip you took me on in the TARDIS, I remember holding you as a baby, I remember looking after you when your parents were ill. I'm as real as she is!"

"I know," I say softly. "But you're not her. I've said my goodbyes, and I have closure on that now. You're just a facade. I'm sorry." I pull my hand away and turn, walking towards the mirror without looking back, tears streaming down my face. As I step back through the mirror, I break down and the Doctor holds me tightly as she closes the portal behind me.