"Right, yes, good," the Doctor snapped back, twisting around to narrow his eyes at her. "And what will I say? 'Jenny, that's the Master, he's another Time Lord and dangerous and yes, he's done some horrible things to people in the past, but you mustn't go near him'? Do you think that will work, Martha? Do you really think so?"

He sighed, the exhaustion and pain coming back in a rush, and turned away again to stare blankly at the wall. "I'm sorry, I really am, and I wish things could be different. But they're not. There are some things I can't tell Jenny, or you, or anyone. It's just the way things are."