Wildlife photographer Pickett Smith (Sam Elliott) embarks on a solo expedition through a picturesque swamp off the coast of Florida, capturing breathtaking images of the local flora and fauna. However, his serene journey is soon marred by pollution, which he diligently documents. Nearby, siblings Clint and Karen Crockett (Adam Roarke, Joan Van Ark) are joyriding in a speedboat. Clint, noticeably inebriated, causes a dangerous near-miss with Pickett's canoe, resulting in the canoe overturning. In a fit of annoyance, Pickett tosses Clint into the water, much to Karen's amusement. They graciously assist in towing Pickett's canoe back to their family estate, owned by their father, Jason Crockett (Ray Milland).
Upon arrival, they invite Pickett to join their extravagant annual family celebration, which is in honor of the 4th of July and several family birthdays, including Jason's. Jason, portrayed as a cantankerous and wealthy patriarch confined to a wheelchair, begrudgingly accepts Pickett into their midst, but a clear rift in ideology emerges; while Pickett reveres nature, Jason harbors a fierce desire to dominate it. The estate, an idyllic retreat, is plagued by an overwhelming population of frogs and toads, which Jason attempts to eliminate using an assortment of pesticides and toxic substances. Proudly deeming himself and his family the "ugly rich," Jason’s relatives display a remarkable inventiveness in their selfishness, with the notable exception of Karen, who shares an undeniable attraction to Pickett.
In a private conversation, Jason persuades Pickett to scout the island to assess the environmental devastation wrought by the pesticides and to keep an eye out for Grover, an employee dispatched to administer more chemicals across the island. As Pickett hikes through the wilderness, he stumbles upon a grim sight: countless lifeless animals, primarily snakes and frogs. Venturing further, he discovers Grover's jeep and ultimately finds Grover himself, lifeless and overwhelmed with writhing, poisonous snakes. Gently using a stick, Pickett clears the snakes away, only to reveal Grover’s mutilated and swollen face – a harbinger of the tragedies to come.
Returning to the estate that evening, Pickett chooses to keep Grover's death a secret, only confiding in Jason, who expresses relief that discussions surrounding it are hushed. The following morning, the Crockett family, undeterred by Grover’s demise, sets about preparing for their grand celebration. However, calamity strikes as Michael Crockett (David Gilliam), Jason’s younger brother, suffers a grotesque fate while attending to a fallen telephone line. In a series of unfortunate accidents, he accidentally wounds himself, immobilizing him beneath a tree festooned with hanging moss, from which a horde of spiders descend to enact their dreadful revenge.
Meanwhile, Iris ([Hollis Irving]), Jason's older sister, sends her son Kenneth (Nicholas Cortland) to the greenhouse for flowers, unaware of the lurking dangers. As Kenneth collects blooms, he accidentally releases a swarm of lizards that knock over poisonous chemical containers, enveloping him in a lethal gas. Shortly after, Iris entices herself into the woods, drawn by the pursuit of a butterfly that she longs to add to her collection. Her innocent endeavor turns monstrous as snakes and an aggressive rattlesnake corner her, leading to her ignominious end.
Meanwhile, Iris's husband, Stuart (George Skaff), narrowly misses discovering his wife’s fate, instead becoming prey to lurking alligators. As horrified family members start to reveal the grim realities of their loved ones, Kenneth's lifeless body sends shockwaves through the group, but Jason, stubborn and impervious to loss, demands the party must go on. Bella ([Judy Pace]), Kenneth's distraught girlfriend, insists on leaving but is met with Jason's ironclad refusal. She eventually escapes with two servants, Charles ([Lance Taylor Sr.]) and Maybelle (Mae Mercer), only to face chaos as Clint takes them across the lake, which culminates fatally when he is viciously attacked by water moccasins.
In the mayhem, Clint's wife Jenny (Lynn Borden), watching helplessly from the shore, becomes entrapped in mud and is swiftly executed by a snapping turtle. With the atmosphere thickening with dread, Pickett and Karen, accompanied by Jenny and Clint’s young children, realize they must flee. As they navigate towards safety, they come across Jenny’s body, grotesquely positioned, and masking the truth of the horrific events. With determination, they paddle across the frigid waters, fending off more serpents and defeating an alligator in their race for survival.
Their harrowing journey leads them to the abandoned belongings of Bella and her companions, suggesting that their fates are ominously unclear. As they make their way to a road for rescue, a passing car provides a glimmer of hope as a mother takes them towards Jefferson City. However, the son’s newfound “friend,” a large bullfrog, serves as a chilling reminder of the peril lurking beyond their sight.
Back at the estate, Jason is left alone, entrapped in his lavish but increasingly claustrophobic mansion. With only his dog, Colonel, for company, he becomes surrounded by a growing legion of frogs as night approaches. He listens to patriotic tunes on his Victrola, trying to block out the impending doom. Yet, as the frogs invade the fragile sanctuary of his home, a paralyzing fear overwhelms him. Unable to cope with the relentless assault of nature, he suffers a catastrophic heart attack, tumbling out of his wheelchair onto the floor. In a poignant twist of fate, nature has finally triumphed over its tormentor. In the haunting final scene, the lights of the mansion extinguish, leaving only the echo of nature's victory as a reminder of humanity's fragile existence.
