DOCTOR: Ah. It's rather as I feared. We're in the Axis.

PERI: The Axis?

DOCTOR: Yes. It's, er, a complex. Look, are you sure you want to hear this? We'll be on our way before...

PERI: Doctor!

DOCTOR: Right, well, er, imagine the spokes of a bicycle wheel. Each spoke is a damaged or truncated reality, a sort of dead-end in Time. They're formed by changes in established history, some big, some relatively small, which cause the flow of subsequent events to spiral out of control. Often catastrophically.