"That depends," River said carefully. She knew as well as anyone how tempting the idea of time travel could be, and how dangerous the consequences were.

"He's very willing to help people in trouble, but...only so far as it doesn't mess with the fabric of time. I mean, he's brilliant - sometimes lousy fashion sense and has the attention span of a hyperactive child, but utterly brilliant when it comes to the sciences and languages. What is it your friends are working on?"