chunk: In the book's fictional universe, history resists time travel that would cause the past to be altered, by preventing visits to certain places or times. Typically the machine used for time travel will refuse to function, rendering the trip impossible. In other cases "slippage", a shift in the exact time target, occurs. The time-traveler arrives at the nearest place-and-time suitable for preventing a paradox; variance can be anything from 5 minutes to 5 years. Some periods theoretically accessible can also be deemed too dangerous for the historians by the authorities controlling time travel.Kivrin Engle, a young historian specializing in medieval history, asks her reluctant instructor, Professor James Dunworthy, and the authorities running the project to send her to Oxford in 1320. This period had previously been thought too dangerous, because it stretched the time travel net 300 years earlier than it had ever been used before. Professor Gilchrist, who took charge of the project in the