In "Planet of the Ood", Donna seemed pretty set on wanting to go home. Until the end of the episode, that is. . . .
I've edited this story to bring it in line with a little theory of mine inspired by the series 4 finale. No spoilers, but I've added a single, shining word :)
Disclaimer: Surprise, surprise, I don't own Doctor Who. Nor do I get anything from writing these stories--except wonderful, constructive reviews! Wink, wink; nudge, nudge ;)
Aliens on earth, okay. Traveling through time, living through ancient history, fine. But this, slave-trading humans on an alien world, with oh, so alien aliens holding their brains in their hands, the heart-wrenching sadness of their song, Donna couldn't take it. The Doctor heard it, "all the time," he'd said, but he's made for this life. In that moment, Donna realized she belonged back on earth. She thought it would be so wonderful to travel among the stars, but this. . . no.
"I wanna go home."
Had the words actually left her lips? They had, and she was serious. And as much as instinct told her to take them back, it also felt right. What good could she really do out here? She couldn't be the companion the Doctor needed. He needed someone as strong as he was; he needed to have Rose back, really. The sooner he dropped her off, the sooner he could go about saving the rest of the universe.
This train of thought was interrupted by a chance to tell off mister what's-his-face for his incredible callousness and stupidity, running for her life again, seeing said mister what's-his-face actually turned into an Ood, then, finally, hearing that beautiful song of the free Ood.
As they were escorted back through the facility to the TARDIS, Donna shied away from the Doctor and let the swarms of grateful Ood surround him. She didn't feel like she'd done all that much to help, and was starting to succumb again to the overpowering need to get back to earth. Suddenly, as the Doctor passed out of sight around the next corner, a thick, golden fog seemed to fill her mind, and she stopped still.
No sooner had it begun than it seemed to vanish, and Donna quickened her pace to join up with the Doctor. What had she been thinking? Leave all this? Give up the adventure, the excitement, the chance to help wherever--and whenever--it was needed? "Stay with the Doctor," a voice in her mind told her. It startled her at first, but she thought the words again, "stay with the Doctor," this time in her own voice. Yes, she should listen to that voice, not the voice of cowardice. What had come over her? Culture shock, she guessed. "Stay with the Doctor," the words came unbidden yet again. I'll stay with the Doctor, Donna thought. Then, when he turned and saw her, and he held out his hand to her, she took it as they came within sight of that magnificent blue box.