Wildlife photographer Pickett Smith (Sam Elliott) is taking photographs of the local flora and fauna as he canoes through a swamp surrounding an island containing the affluent mansion estate of the wealthy and influential Crockett family. Also evident throughout the swamp are numerous indicators of pollution, which Pickett believes are connected to pesticide use on the island plantation. After Clint Crockett (Adam Roarke) accidentally tips over Smith's canoe, he and his sister Karen (Joan Van Ark) escort Smith to the family mansion where he meets the entire Crockett family. The grouchy, wheelchair-bound patriarch Jason (Ray Milland) intends on spending the next day enjoying both the 4th of July and his own birthday celebrations uninterrupted. Due to the mutual dislike of the fauna around the mansion, Jason sends a man named Grover to spray pesticide in order to get rid of the amphibians. Pickett later discovers Grover's corpse covered in snake bites in the swamp not far from the house. Despite this warning, Jason continues with his celebrations the next day.
Early next morning, Michael Martindale (David Gilliam) sets out to check on a possible downed telephone line. He accidentally shoots himself in the leg, and is rendered immobile by strange white moss hanging down from the surrounding trees. Tarantulas descend from the branches and kill him.
Back on the estate, Jason's sister, Iris Martindale (Hollis Irving) sends her son, Kenneth (Nicholas Cortland) into the greenhouse to collect flowers for a centerpiece. As he gathers the flowers, he fails to notice dozens of lizards entering behind him. The lizards swarm over the stacked shelves, knocking over numerous jars of poisonous chemicals, and the resulting toxic gas asphyxiates him.
Seeing the danger posed by the animals, Pickett suggests that everyone should leave the island, but Jason is adamant that nothing will ruin his day.
In the meantime, while chasing after a butterfly, Iris is frightened by snakes along her path, and in a panic, falls into a swamp, where leeches latch on to her. She manages to detach a few, but fatigued, she falls down near a rattlesnake, which promptly bites and kills her. Her husband, Stuart (George Skaff), comes looking for her, but falls into the swamp and is eaten by alligators.
On Pickett's advice, Charles (Lance Taylor, Sr.) and Maybelle (Mae Mercer) Jason's long-suffering butler and cook, decide to leave, along with Kenneth's fiancee, Bella Garrington (Judy Pace). Clint takes them across the lake in his speedboat. Clint stays behind and searches the nearby grocery store while the others walk on. A flock of birds suddenly appear as they run off behind a building. Their fate is left unknown (though a strewn-open suitcase is seen later). Clint discovers his boat is untethered and swims to reach it, but a water moccasin kills him in the water. His wife, Jenny (Lynn Borden), tries to rescue him but gets stuck in the lake mud and is killed by a large snapping turtle.
Karen and Pickett decide to leave with Clint's and Jenny's children, leaving Jason behind as he refuses to join them. They cross the lake in Pickett's canoe, encountering an alligator and water snakes, which Pickett dispatches with the boat paddle and a shotgun. They eventually make it ashore and reach a road, where they hitch a ride with a woman (Carolyn Fitzsimmons) and her son (Robert Sanders). She tells them that she is heading to Jefferson City and has not seen a single person or car on the road all day, while the boy shows them a huge frog he took from summer camp.
Later that night, Jason, now alone in his mansion (save for his dog Colonel), witnesses hundreds of frogs breaking into the house and staring at him. Looking around the room at his stuffed animal trophies adds to his tension and he falls out of his wheelchair and collapses, apparently dead, the frogs croaking as they hop over his corpse.
Finally, all the lights in the mansion completely flicker out.
Frogs received a score of 29% on Rotten Tomatoes.
A reviewer from HorrorNews.net found it odd for a horror film to be titled Frogs when all the killings in the film are done by other animals, and discussed the acting: "Sam Elliott is good as always. He manages to feel like the outsider while also feeling like part of the group. It makes his role work in ways that it might not work in someone else's control. Ray Milland is also fairly good as the patriarch of the Crockett family. He personifies that bullheaded 'you listen to me because I’m always right' attitude in such a believable manner that you think he is that guy. The rest of the cast isn't as great as these two, but their lack of good performance only helps to make their deaths more fun to watch. They overact or underact in the perfect ways to make the movie priceless".
The film was given a PG rating by the MPAA.