Youngman Duran, a deputy on a Hopi Indian reservation in New Mexico, begins to investigate a series of mysterious cattle mutilations. Abner Tasupi, an ancient and embittered medicine man who raised Youngman after his parents died, tells him he has cast a spell to end the world that very night, but Youngman assumes he simply is babbling while under the influence of datura root. The following morning, Youngman finds Abner's bloodless body on the floor of his shack, and nearby he discovers a dead shepherd and most of his flock.
Tribal Council chairman Walker Chee has discovered a stratum of oil shales in Maskai Canyon, the most sacred ground in the tribe's domain. Walker is dynamiting the caves in an effort to unleash oil, and is planning to sell the rights to process them to the tycoon Roger Piggott of Peabody Oil. Walker is desperate to keep word of the attacks from leaking to the media before he completes the deal.
Although common sense tells him otherwise, Youngman's faith in tribal beliefs and superstitions leads him to suspect the unexplained deaths may be connected to the spell Abner claimed he cast. British scientist Philip Payne is certain they are the work of vampire bats infected with bubonic plague. As they spread throughout the area, swarming through a missionary group's campsite and infecting everyone in their path, Philip and Youngman join forces with Anne Dillon, a young white medical student who runs a ramshackle clinic on the reservation and is in love with Youngman, to track the bats to their lair and destroy them.