"No one knows how or why, and trust me, we've been trying to work it out. My guess it's you who vanished when you stepped out of the TARDIS. I came here walking through an ordinary office door, myself. Don't worry, there's every indication from the people who have been here, gone and come back, that you'll be dropped back at the exact point of time and place you left. Almost certaintly with no memory of being here."