It is January, and Park Ranger Anna Pigeon is sent to Isle Royale in Lake Superior to learn about managing and understanding wolves, as her home base of Rocky Mountain National Park might soon have its own pack of the magnificent, much-maligned animals. She's lodging in the island's bunkhouse with the famed wolf study team, along with two scientists from Homeland Security, who are assessing the study with an eye to opening the park in the winters'an act that would effectively bring an end to the fifty-year project'so that the area can be manned to secure the scrap of border with Canada.

Soon after Anna's arrival, the wolf packs under observation begin to behave in peculiar ways. Giant wolf prints are found, and Anna spies the form of a great wolf from a surveillance plane. The discovery of wolf scat containing alien DNA leads the group to believe that perhaps a wolf/dog hybrid has been introduced to the island. When a female member of the team is savaged, Anna is convinced she is being stalked, and what was once a beautiful, idyllic refuge becomes a place of unnatural occurrences and danger beyond the ordinary. Alone on an island without electricity or running water, with temperatures hovering around zero both day and night, Anna fights not only for the wolves but for her own survival as well.