The Doctor followed behind Billy, catching up enough to walk next to him. He did wonder whether the TARDIS might be able to get into this chamber, but if she could it might be better not to test the theory until it became necessary.

"Oh, you know. What wise old aliens talk to each other about when they catch up. Weather, sunspots, some jokes about UFOs, figure out our degrees of separation to Kevin Bacon. Mine is zero, by the way. I call him bacon bits. Nice bloke, really."