Ray Milland plays an aging, grumpy, physically disabled millionaire who invites his family to his island estate for his birthday celebration. Sam Elliot plays a free-lance photographer who is doing a pollution layout for an ecology magazine. Jason Crokett (Milland) hates nature, poisoning anything that crawls on his property. On the night of his birthday the frogs and other members of nature begin to pay Milland back.

Freelance photographer Pickett Smith is canoeing through a swamp in the South, taking pictures of nature and the damage done to the environment by trash and pollution when he paddles onto a large lake and is capsized by a motorboat driven by intoxicated Clint Crockett. Clint and his sister Karen are very apologetic and offer to take Pickett to the shore to dry out at the island mansion of their domineering grandfather, Jason Crockett. The family is staying at the mansion to celebrate the Fourth of July as well as Crockett's birthday. Along with Karen and Clint, the gathering includes Jason's daughter, Iris Martindale, her husband Stuart and son Kenneth, Jason's grandson Michael Crockett, Clint's wife Jenny, their two young children, Tina and Jay, Kenneth's black girl friend, Bella Berenson, and servants Maybelle and Charles. At lunch, Pickett observes the tension among various family members and Jason, who has been confined to a wheelchair for many years and knows that his only hold on the family is his money. Jason and others relate that, for the past few nights, the increasingly visible frog population has been making so much noise that they cannot sleep. Because Pickett specializes in ecological photography, he offers to look through the grounds and investigate. Later, Karen, the only family member who both shows affection for and stands up to Jason, tells Pickett that he has impressed her grandfather. That afternoon, Jason summons Pickett and privately tells him that he would like him to investigate the disappearance of Grover, an employee who earlier left the mansion in a Jeep and has not returned. Jason believes that Grover has a girl friend across the lake and is annoyed by his absence. When Pickett goes through the woods he comes upon the abandoned Jeep near empty pesticide cans, dead birds and trash. A few moments later, he finds Grover's body covered in snakes and frogs, with his face partially eaten away. Back at the house, Bella offers Maybelle a glass of wine in the dining room and the two black women laugh together when Bella, who is a sophisticated model and designer, confesses that her real name is also Maybelle and she, too, is from the South. That evening, as Clint makes Jenny jealous by dancing and flirting with Bella, Iris laments all of the expensive changes that Stuart has had to make at their mill to comply with new anti-pollution laws. After Pickett drives the Jeep back to the mansion, he privately tells Jason what he has found. Jason is impressed with Pickett's discretion and asks him not to tell anyone else because the Fourth of July week he spends with his family is the only thing he looks forward to and he does not want to spoil it. While they are talking, a number of large frogs throw themselves against the house. Just then, Maybelle screams, summoning everyone to the dining room where a snake has entwined itself onto the chandelier. Jason calmly pulls out a pistol and shoots the snake, then orders Charles to serve dinner. During the meal, Pickett wonders aloud if nature is trying to "get back at us" for ruining the environment. The next morning, after a long night of loud noises made by a burgeoning number of frogs, the family prepares for Fourth of July and birthday festivities. Tina and Jay are anxious to start the games and set off fireworks in the woods while an already drunk Clint, who was a star athlete in college, challenges his effete cousin Michael to a game of "king of the log." Seeing Michael's frustration when he falls off the log, Jason asks him to take the Jeep and drive through the island to find any downed telephone lines that may have caused the phones to stop working. Happy with his responsibility, Michael takes off through the woods, then stops and shoots at some birds flying overhead. Walking on, Michael trips and accidentally shoots himself in the leg. As he falls to the ground, Michael is overcome by tarantulas, which repeatedly bite him until he dies, screaming in pain. Although Pickett passes by a short time later, he does not see the body as it has been covered in spider webs. Back at the mansion, when Iris sees a beautiful butterfly, she asks Kenneth to retrieve the flowers she needs from the greenhouse so that she can catch the butterfly with her net. While she wanders through the woods, Kenneth goes into the greenhouse, where he is killed by toxic fumes released when lizards and frogs break bottles of poison by pushing them off high shelves onto the ground. A few minutes later, Pickett goes to the greenhouse and finds the body, which has been overrun by lizards. Now Jenny becomes hysterical and insists that she wants to leave. Jason refuses to listen and is impatient to start the celebration, so he sends Stuart into the woods to find Iris. Within minutes, Iris dies after being attacked by leeches and snakes, and Stuart is killed by alligators. Although the others do not yet know what has happened to Iris and Stuart, Karen tells her grandfather that they should leave because of Kenneth's death and is even more determined when Pickett divulges that Grover was also killed. Now angry, Pickett insists that they all need to leave the island because something is happening in nature and doubts that they will ever find Michael, Stuart or Iris. Charles and Maybelle also plead with Jason, but he angrily snaps at them for disloyalty and says that they can leave, with Bella, but family members must stay. Clint then takes Bella, Charles and Maybelle across the lake in his speedboat, but when they dock on the mainland, they cannot find the man who runs the small waterfront store. Frightened as large birds take aim at them, Bella, Maybelle and Charles rush off toward a shack while Clint goes back to his boat. Finding that the moorings have been unraveled, Clint dives into the water and swims toward the drifting boat. While Pickett is gathering weapons and ammunition at the mansion, Jenny takes her binoculars and observes Clint swimming toward his boat. Hysterical when she sees water snakes attack him, she rushes down to the shore and is killed by an alligator turtle just as Clint is killed by the snakes. By now even Jason has become worried but still will not leave, and lashes out at Karen, who had always been his favorite. He grudgingly allows her, Tina and Jay to leave with Pickett and tells Pickett that he will be able to take care of himself. Pickett, Karen and the children then silently paddle across the lake, with Pickett fighting off attacking snakes. When they reach the store, Pickett sees the scattered suitcases and clothing of Charles, Bella and Maybelle and realizes that they, too, have been killed, but rushes the children past the debris. Pickett then flags down a woman driving a station wagon on the highway. As her son Bobby shows Tina and Jay the new frog he has found, his mother relates that they have not seen anyone else for hours. That night, as Jason sits alone, drinking scotch in his wheelchair, he looks at the big game heads on his wall and imagines them wailing. Hundreds of large frogs start to break the windows of his study, and he is soon forced to the ground, screaming in pain as the frogs overcome him.