The film examines a troubling environmental crisis: the depletion of the Earth’s ozone layer driven by aerosols has begun to flood high-altitude environments with intense UV radiation. Scientists warn that animals above about 5,000 feet become unusually aggressive, and a small-town sheriff narrowly escapes a deadly mauling by rats as authorities order the evacuation and quarantine of all settlements above that altitude.
Against this perilous backdrop, a group of tourists in Northern California sets out on a wilderness trek, led by tour operator Steve Buckner and Native American guide Santee. The party includes the bickering Mandy Young and Frank Young, Mrs. Shirley Goodwyn and her son John Goodwyn, Paul Jenson, the scholarly Prof. Taylor MacGregor, the team of Bob Denning and Beth Hughes, Terry Marsh, and Roy Moore.
With no outside contact, the hikers remain blissfully unaware of the strange animal activity burgeoning around them. A wolf attack at their camp unsettles the group, but they press on. Mandy and Frank eventually descend from the higher terrain, only to be struck again by danger as hawks dive in and Mandy is killed. Frank survives while wandering through the forest until he happens upon a little girl, abandoned when the government quarantined her town, and together they begin a tense trek toward safety.
Meanwhile, the balance of the party discovers that their helicopter-dropped food cache has been raided by animals, forcing them to camp again under the threat of further predation. The group’s dangers multiply when a night-time attack by mountain lions imperils them. The tense mood culminates when Jenson, now in charge, brutally pushes the others to keep moving in search of help, dragging along Mrs. Goodwyn, John, and Beth.
The conflict within Jenson reaches a brutal climax when he murders Bob amid rising fear and pressure. A confrontation with a bear ends in tragedy for Jenson as he is fatally mauled while attempting to assault Beth, leaving the remaining members to flee in the aftermath—Mrs. Goodwyn, John, and Beth taking refuge inside an abandoned helicopter as wild dogs close in. The rest of the hikers press on toward a lower-altitude town, only to find it deserted.
Another peril unfolds as a pack of wild dogs and venomous dangers threaten the group at the river’s edge. Roy and the professor are killed by a pack of German shepherds, and Buckner leads the survivors toward a safer path by rafting down a nearby stream. By dawn, Beth, Mrs. Goodwyn, and John awaken inside the downed helicopter while a rescue helicopter and army personnel in hazmat gear appear on the horizon. They encounter a living little girl who is brought to safety by the soldiers, marking a hopeful turning point as Buckner, Terry, and Santee reach a Ranger station after floating downriver.
In the end, most of the transformed wildlife have perished, victims of the very solar radiation that once drove them mad. The survivors are saved, and a sense of fragile calm returns as the film closes with one final, eerie image—a hawk lunging toward the screen just before the credits roll.
