After Donna's been kidnapped by the pilot fish Santabots, there's an excellent scene in which the TARDIS physically comes bouncing down the highway, careening after her. They showed this at Children In Need; I watched it on the web, and it was amazing: the TARDIS comes careening down the motorway, music so crazy you couldn't really hear what was going on, on the screen. But in the crowd: everybody in the whole concert hall, gasping at once, breathing with a single breath. It was huge. The moment made me understand Doctor Who fandom a whole lot more. Think about the power of that moment: when the TARDIS comes into the shot, and the whole world starts cheering, all of them in love with exactly the same thing, at exactly the same time. And from one kidnapper to another, the Doctor asks Donna to jump. She asks if his "friend," whose name he hasn't yet said, trusted him too. Did she jump? Did she ever.
Being a normal person -- a person who didn't notice the Santa Fish or the Slitheen or any of that shit -- doesn't make you less. It makes you Donna, which you already were. "The Oncoming Storm" isn't bullshit mythology, it's the way of things. But just like the Time War, which hurt everybody but the normal people, the Doctor comes in waves. There's a trees/forest analogy running through the season, and Donna starts it here. The trees are made of forest, and the forest is made of trees. No trees, no forest: but without a forest, the wind can take you down, one by one.
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.