When one of the Grand Canyon's mules is found stabbed to death at the bottom of the canyon --- and its rider missing --- FBI agent Atlee Pine is called in to investigate. It soon seems clear that the lost tourist had something more clandestine than sightseeing in mind. But just as Pine begins to put together clues pointing to a terrifying plot, she's abruptly called off the case. If she disobeys direct orders by continuing to search for the missing man, it will mean the end of her career. But unless Pine keeps working the case and discovers the truth, it could spell the very end of democracy in America as we know it.