The film opens with helicopter pilot and guide Don Stober (Andrew Prine) flying individuals above the trees of a vast National Park. He states that the woods are untouched and remain much as they did during the time when the Native American lived there.
Two female hikers are breaking camp when one of them is attacked and killed by a bear. The second woman finds apparent safety within a nearby cabin until the bear tears down a wall to reach her. The National Park's Chief Ranger Michael Kelly (Christopher George) and photographer Allison Corwin (Joan McCall), daughter of the park's restaurant owner, decide to follow a Ranger to the primitive campsite to find the two female hikers. They discover the woman's mangled body inside the destroyed cabin. Allison stumbles across the remains of the first woman while photographing the search.
At the hospital, a doctor tells Kelly that the women were killed by a bear. The Park Supervisor Charley Kittridge (Joe Dorsey) blames Kelly, saying the bears were supposed to have been moved from the park by he and Naturalist Arthur Scott (Richard Jaeckel) before the tourist season began. Kelly and Kittridge argue over closing the park, and decide to move all hikers off the park's mountain while allowing campers to remain in the lowlands. Kelly calls Scott, who says all bears are accounted for and this specific bear must be unknown to the forest.
While searching the mountain, a female Ranger decides to go swimming near a waterfall where she is attacked and killed by the bear. Kelly recruits the helicopter pilot Stober to assist in the search. Flying above the forest, they see what they believe to be an animal, only to discover the Naturalist Scott adorned in an animal skin while tracking the bear. He informs them the animal they are looking for is a prehistoric grizzly bear and at least 15 feet tall. Kelly and Stober scoff at the notion.
At the busy lowland campground, the grizzly tears down a tent and kills a woman. Kelly once again insists on closing the park, but Kittridge refuses. The attacks are becoming a national news story and to counteract this, Kittridge allows amateur hunters into the forest. Kelly, Stober and Scott, now a team, are disgusted by this development. Later, a lone hunter is chased by the bear but he evades the animal on foot, falling into a river and floating to safety. A Ranger at a fire lookout tower on the mountain is attacked by the grizzly, the animal tearing down the structure and killing the Ranger.
Kelly and Kittridge continue to argue over closing the park. Frustrated by the politics of the situation, Scott sneaks away to track the grizzly on his own. On the outskirts of the National Park, a mother and child living in a cabin are attacked by the grizzly. The mother is killed and the child survives, but is severely mutilated. Stunned by this development, Kittridge finally allows Kelly to close the park and ban all hunters.
Stober and Kelly now go after the elusive grizzly alone, setting up a trap by hanging a deer carcass from a tree. The grizzly goes for the bait and the men chase the animal through the woods. When they return, they discover the grizzly has tricked them and taken the deer carcass. Tracking on horseback, Scott finds the remains of the carcass and calls Stober and Kelly on the radio. He is going to drag the deer behind his horse and create a trap by leading the grizzly towards them. The grizzly surprises Scott, killing his horse and knocking him unconscious. Scott awakens to find himself alive, but half-buried in the ground. The grizzly immediately returns and kills him.
Kelly and Stober discover Scott's body and in despair, decide to return to the helicopter to find the grizzly from the air. They immediately spot the bear in a clearing and quickly land. The grizzly attacks the helicopter, swiping the craft causing Stober to be thrown clear. The grizzly kills Stober and then turns on Kelly, who frantically pulls a bazooka from the helicopter. Before the bear can reach him, Kelly fires the bazooka at the grizzly, killing the animal instantly. For several seconds, Kelly sadly stares at the burning remains of the grizzly and then walks towards Stober's body.