Kent Horvath, Michael Shanks, is a scientist who hones a new pesticide aimed at yellow jacket wasps. He tests it on eight wasps and discovers it kills six, but the two survivors end up with extremely high pheromone levels that trigger their brutal aggression. That night, a janitor accidentally releases them, and they attack, killing him. The next morning, his body is found alongside one of the wasps, while the other remains nowhere to be found.
In the morgue, the mortician hears a buzzing from the janitor’s mouth, which releases the second wasp and seals the mortician’s fate as the insect stings him to death before flying away. Cristina Brown, Carol Alt, an entomologist, leads a class about bees and explains how an attacking insect releases an attack pheromone that signals others of their kind to swarm. The class is interrupted when she is called to investigate the mortuary death. Agent Doug Heydon, Booth Savage, informs her about the janitor’s death and Kent’s research, prompting a deeper look for a possible link.
Q, Richard Chevolleau, Kent’s exterminator friend, is summoned to a home plagued by a yellow jacket infestation. With limited pesticides on hand, he goes to Kent for a restock. Kent warns him not to use the experimental formula, but Q mixes it with the regular solution. When he sprays the nest, venom analyses reveal a mutation that makes the wasps’ venom far more lethal and their aggression far more intense.
Meanwhile, the Chief of Police, Bill Lake, uses blackmail to pressure Mayor Gibson, Christopher Bondy, over secret meetings with Phineas Washburn, the owner of Washburn’s Tasty Sauce. The Chief hints that he’ll reveal the covert dealings unless a payoff is made, leaving the mayor seething.
Back at his workshop, Q stores the treated nest in a trash compactor, but about a quarter of the nest survives, including the queen. They chase him to his truck, where he manages to administer antivenom before passing out. The remaining wasps disperse, releasing attack pheromones that attract swarms from other areas and mutate them to carry venom as potent as the sprayed nest. Cristina and Kent soon visit the home of a man who was stung to death after the swarm followed raw meat patties on his grill, and they find a dead wasp on his clothes to bring back to the lab for analysis. Meanwhile, Rafe, Jonathan Malen, the teenager under Q’s supervision for the summer, wakes him, and they agree to hunt down the remaining portion of the swarm themselves.
At the lab, venom tests confirm the lethality matches the event that killed the janitor. After informing Agent Heydon, they conclude that the annual burger cook-off will act as a magnet, drawing the swarm into the heart of the city. The lone wasp that first killed the janitor reaches Mayor Gibson’s office and kills him by stinging his eye. His assistant, Ellie Martin, Ellen Dubin, discovers the body and chases the wasp with a double-barreled shotgun, ultimately killing it.
Heydon returns to warn the Chief, urging them to cancel the cook-off, but neither authority believes the threat and they threaten legal action instead. Soon after, the Chief is killed when the wasps are drawn to his rare steak. Q and Rafe work to track the lingering portion of the swarm with a pheromone detector, dragging the main cluster toward a garage. Q uses a flamethrower to clear the swarm, though part of his van melts in the blaze.
The cook-off begins, and Heydon arrives to press Washburn, Tim Thomerson, to halt the event. Before he can act, the swarm assaults the crowd, killing a fierce reporter, Melanie Sheer, Maria Brooks, while her cameraman, Chuey, Balázs Koós, records from a phone booth. Washburn tries to hide in a dunk tank, but two wasps crawl along the hose and sting him in the mouth.
Kent, Cristina, and Rafe move quickly to administer antivenom to the stung, using tire-patching spray to seal the breathing holes of the wasps. Q joins them and lures the wasps toward a golf cart, hoping to trap them inside a garage. They gather petrol tanks and gasoline, planning to destroy the swarm once it’s contained. Q manages to corral the insects into the garage and ignites the fuel with Rafe’s lighter. The fire wipes out the swarm, but the queen survives the blaze and begins laying eggs.
In the aftermath, Kent and Cristina start dating and share a quiet dinner, while the shocking twist reveals that the queen has survived the inferno and is already in the process of laying eggs, signaling that the threat may not be over. The film closes on that uneasy note, with the swarm’s resilience hinted at for the future.
