Time Pressure — Spider Robinson

A STRANGE BLUE LIGHT
... has brought a visitor to an American commune in Nova Scotia. The visitor's name is Rachel, a beautiful woman from another civilization ... and another time. Her one-way mission is simple: collect data on the past of the human race:

But why does she risk immortality for this mission? Why are her methods so seductive, so devastating? And will this emissary from the future destroy Earth's past?

Only time will tell.