The Doctor shuddered. "Please don't," he said automatically, and then realised how rude that sounded. Yet again, rude and not ginger. "Rose tried to do that once. To change history. When you change established events, at least in the rea--where I'm from," he amended, having to remind himself that Tonks' world was very real to her, even if it wasn't to him. "That's when Reapers show up, and everything goes to pot."

"They feed on time distortion," he explained. "And there's no stopping them. I've been down the gullet of one of those things, Tonks. I don't want that to happen to your world." The Doctor sighed and shook his head. "I'd love nothing more than to tell you that you can change things, and everything will come out rosy. I would love that. And maybe the rules are different where you're from. But would you risk that? Everyone you know and love...Every place that was ever important to you...All destroyed."