You Are Not Alone
The Doctor gives Donna a ring to keep her hidden from the pilot fish, but soon figures out that her disappearance arose from her body's saturation by huon particles, a source of energy that hasn't existed for billions of years, and which was being manufactured again by Torchwood, in its base below the Thames. It's something so powerful that Gallifrey was afraid of it, and Torchwood thought it could handle it. She was getting them in her coffee, and they can't be biodamped by a simple ring. Which is to say, the thing she thought would make her special is now the thing that can't stop her from being special. Which is to say, she's not a replacement Companion; she's a mirror: The Last Of The Singletons, so desperate to hear those four little words that she can't see the danger signs. So afraid of being alone.
In contrast to the cuppa that saved Ten and the world a year ago, the huon was administered through Donna's coffee. Later, the Doctor takes Donna back 4.6 billion years, when the Earth was waiting to form: a Rachnoss ship drifts into the system and begins to pull debris around itself, attracting dust and light and growing mass. I mean to say that at the center of the Earth is a secret: something dark, scary, chthonic, something forgotten, something you can't wrap your head around. Something that's scary because you can't understand it, because it looks ugly to you, because you don't have enough information, because you're not comfortable: something that makes you shudder without knowing why. Does it make you love Earth less? It shouldn't.
When the Doctor enters, dressed as a Santa, the Empress of the Rachnoss immediately sees through the disguise, of course: you can't do seemings in front of stuff from the depths. They're about the ugly truth, about teeth and blood; they don't have the imagination to be fooled. The Doctor makes the Empress an offer: he'll find a planet for her and her children, if she ends it. If she and her children give up their birthright, if they agree to give up what is theirs, he will relent. But shadows can't grow brighter by the shouting of the light, so the Empress declines. What happens next is her own doing, the Doctor impresses upon her, and he tells her his world. It is far away and long since gone, but its name lives on. Gallifrey. Unspoken Gallifrey, silver-leafed trees and burnt orange sky, deep red grass. Gallifrey, the shining world of the Seven Systems. Gallifrey of the Untempered Schism. Gallifrey, the murderers of the Rachnoss. The Empress screams in rage and fear, the oldest enemy back again, killing her children. The Doctor takes some ornament bombs and tosses them into the walls and pipes, bringing the river into the base, into the pit. The room is full of fire and flood, and the Empress screams. Donna begs him to stop, tries to give him peace, but there's no peace here. Just rage. Rage, because there's one planet in all places and times that brings him back, that gives him peace and makes him feel something. Now that Gallifrey's gone, it's home. And at the center of that planet, it turns out, is a secret. Something dark and scary, something forgotten, something you can't wrap your head around because you don't have enough information. If I give you something that looks like this