At Murphy's Hotel in an alpine village somewhere in Northern California, Steve Buckner (Christopher George) prepares to board a dozen hikers into two helicopters to fly up the mountain to Sugar Meadow, where they will begin a days-long nature hike. Local ranger Chico Tucker (Walter Barnes) privately tells Steve that there have been all kinds of accidents lately and maybe this hike isn't a good idea, but Steve refuses to call it off.Later that day, after a brisk trek, the hikers rest on several large rocks where the group is introduced; Professor MacGregor (Richard Jaeckel) takes photographs of eagles and hawks. Frank and Mandy Young (Joe Cedar and Susan Backlinie), a young married couple, confess that they are taking this hike to pull together their shaky marriage. An older woman, Shirley Goodwyn (Ruth Roman), admits that she's only there because her son, Johnny (Bobby Porter), wanted to come, and already she wishes she had stayed home in Beverly Hills. Paul Jenson (Leslie Nielsen) is an advertising executive from Los Angeles with an angry, derisive and prejudiced sense of humor. Bob Dennins (Andrew Stevens) and Beth Hughes (Kathleen Bracken) are teenagers in love. Roy Moore (Paul Mantee) is a former professional football player sidelined by cancer. Terry Marsh (Lynda Day George) is a television reporter. The steadiest person among them is Daniel Santee (Michael Ansara), a Native American.During a rest stop, Daniel stands apart from the others, studying the surroundings. Finally, he wonders out loud why everything is so quiet. Other than the wind and the caws from predatory birds, the hills are silent. Jenson, who calls Daniels "Kemo Sabe," says there's nothing to worry about, but several birds, including an owl and a hawk, land next to Johnny, scaring the boy and his mother. Steve and Daniel say they have never seen birds act like that before.In the restaurant of Murphy's Hotel, Ranger Tucker sits down with his friend Burt, the local sheriff, and tells him there's been a spate of rattlesnake bites. At that moment a reporter on the television set over the bar says a White House bulletin is claiming that chemical waste released into the atmosphere has dangerously depleted the ozone layer, which protects all life from the sun's radiation. Tucker admits that the sun has been "peculiar" lately, but dismisses the report as "ozone crap."On the mountain, the hikers stumble upon a camp where a fire is burning and coffee cups are ready to be filled, but nobody is around. Steve says the campers will soon be back. He leads the hikers to a nearby spot to bed down for the night. They build a fire, and while several of the hikers sing campfire songs, Daniel pulls Steve aside to tell him that something strange is going on in the woods. Steve asks him not to say anything to panic the others. The two decide to take turns standing guard. That night, as Terry wonders why the other campers haven't returned, several wolves attack Mandy in her sleeping bag. The campers chase them off, but Mandy's hand has been badly bitten and she needs to get to a hospital.At daybreak, Mandy and Frank leave the others and hike to a nearby ranger tower to call for a helicopter, but birds gather in the trees and circle overhead. Suddenly, hawks swoop down and attack her, and before Frank can chase them off, Mandy falls over a cliff to her death.Meanwhile, as the rest of the hikers continue down the mountain, Johnny picks up snatches of radio reports about an ozone emergency resulting in a chemical imbalance in the forest. When Johnny alarms the other hikers, Shirley shouts at him and accidentally knocks his radio into a creek. When the hikers reach a spot where food has been left for them, the boxes have been ripped apart by raiding animals and nothing is left. Jenson, challenging Steve's competence, says the group should stay there and wait for a helicopter to return, but Steve insists on pushing on down the mountain.Meanwhile, Frank is wading through a creek when he finds a little girl standing on the bank. Frank asks the girl where her parents are, but she is in shock and doesn't react to him until a hawk swoops down and makes her scream. Frank picks her up and carries her.Late that night, Ranger Tucker is awakened by the telephone. Burt tells him the National Guard is in town to evacuate everybody above 5,000 feet where the radiation is the strongest and is making all animals go crazy and attack people. As Tucker hangs up, he hears something rattling and gnawing. He turns on the kitchen light, finds the room empty, and gets a plate of chicken out of the refrigerator. But as Tucker goes into a drawer for a knife, some rats jump onto the table. Tucker tries to stab them, but a couple of rats leap on him and tear his face. Tucker runs upstairs to wake his wife, Rita. They hurry outside and get into their car before several vicious dogs can attack them.At the camp, after wolves attack the hikers again and injure Daniel, Jenson (who is clearly growing more crazy due to the radiation now affecting him) says he's going to walk back up the mountain to the ranger tower, which is closer than the village, and talks Shirley, Johnny, Bob and Beth into going with him. The others continue down the mountain. That night, as lightning flashes and rain pours, Jenson suddenly becomes violent. He calls Shirley a "Beverly Hills bitch" for no reason and threatens to toss Johnny off a cliff. Bob and Beth realize they've made a mistake by coming with Jenson as he is the only human now affected by the sun's radiation, but before they can leave, Jenson kills Bob with a blow from his walking stick and drags Beth away from Shirley and Johnny. While Jenson tries to rape the teenager, however, a bear appears and Jenson wrestles the bear, which kills him by biting his neck out. Shirley and Johnny grab Beth and run away, leaving Jenson for dead.In the morning, after a night of walking, Frank and the little girl arrive in the village where they started, but everything is quiet, after the evacuation. Houses are empty and businesses are closed. A truck is found but ridden with snakes. When Frank opens the door of an abandoned Army truck, a dead man falls out with fang marks on his face. Outside Murphy's Hotel, a mad dog approaches them. Frank puts the girl inside an SUV and grabs a hammer from a toolbox inside. Attempts to start up the car fail, as it won't start up; the keys having been left behind by its owner. Frank tells her he's going to make a run for his car not far away, and no matter what happens she must stay where she is until he returns. Frank keeps the dog at bay with the hammer. As soon as Frank reaches his car and opens the passenger door, several rattlesnakes inside bite him. This results in the dog attacking Frank and killing him.Meanwhile, Shirley, Johnny and Beth take sanctuary in a grounded Park Ranger helicopter whose pilot has been killed by a pack of dogs that proceed to attack the three. Beth barely survives the attack.Meanwhile, Steve's group is attacked by a another pack of dogs at a camp of dilapidated cabins. Professor MacGregor and Roy, badly mauled as they struggle with the dogs, yell for Steve, Terry and Daniel to run and save themselves. The three hurry down to the nearby creek and push a raft into the water, but as they push off, the dogs leap onto the raft, forcing Steve, Terry and Daniel overboard. The three hang on as a current catches the raft and pulls it downstream through rapids.When Shirley, Johnny and Beth wake up hours later, everything is quiet and the dogs are all dead. As Johnny and Shirley step out of the chopper, they hear another helicopter coming and wave their hands as it approaches.In town, soldiers in hazardous-material suits approach Murphy's Hotel. Dead birds and animals lay everywhere, killed by the very same radiation that drove them crazy. Corpses of looters are found too, having been presumably killed by the animals. Four of the soldiers see the little girl hiding inside the same SUV Frank left her in, and rescue her.Not far away, Steve, Terry and Daniel are sleeping on the drifting raft when they hear voices and a distant siren. Looking up, they see a dozen people standing on a bridge, welcoming them back to the normal world.In the final shot, a surviving eagle flies at the camera.