An alien spacecraft crashes in a U.S. national park, and a mosquito drawn to the pilot’s lifeless body sucks its blood, mutating into an abnormally large, ravenous insect. As a couple headed to a summer camp in the park, Ray, Tim Lovelace and Megan, Rachel Loiselle, pull over after the encounter and discover the mosquito’s severed proboscis lodged in the car’s radiator. At the same time, Chief Morrow, Guy Sanville, the park’s authority, orders the use of a gaseous mosquito repellent, hoping to curb what seems to be a localized threat. The tension in the park rises quickly as the situation spirals from a few isolated incidents into a full-blown outbreak.
In short order, three bank robbers—Earl, Junior, and Rex—arrive in the area, transforming the crisis into something far bloodier. Rex, seeking shelter in an outhouse, confronts a gigantic mosquito and screams for help as Junior accidentally fires a shot that fatally wounds him. Earl then shoots and kills the mosquito with a shotgun, a moment that alters the criminals’ fates and foreshadows the chaos to come. As the day unfolds, two park rangers are fishing when they are torn apart by swarms of biting insects. One ranger loses an eye to a brutal bite, while the other fights for his life in the water, only to be stabbed by a mosquito upon reaching shore. The park’s calm gives way to horror, and night brings more terrors as a couple in a tent is viciously attacked when a mosquito slips inside to impale the woman.
The next morning, Ray meets Parks, Steve Dixon, a meteorologist who has come into the park to search for a rumored meteorite. They drive together toward the park’s heart, hoping to find help, and Parks’ radiation monitor suddenly spikes, pointing toward a boat where the dead ranger’s body lies. They head to the police with the grim discovery, only to find that nearly everyone in the park—rangers and visitors alike—has been killed. Megan identifies the strangest evidence, a dead mosquito turned into a specimen of Aedes aegypti, and the group learns that the park has lost much of its infrastructure: electrical and communications systems are down, and the world outside seems unreachable.
The survivors decide to escape the danger by fleeing in an RV, but they are soon intercepted by Earl and Junior, who are taken captive and tied up. A relentless swarm pursues the vehicle, battering the windows and threatening the occupants. In a moment of desperate courage, Megan stabs Earl with a severed proboscis, while another wave of mosquitoes presses closer. The RV swerves into a crash, and the group crawls through a pipe to dodge the buzzing horde. Inside the pipes, Junior is fatally overwhelmed as a swarm closes in, his eyes bulging in a grotesque display of horror.
The chase continues as the group splits to search for answers: Ray, Earl, and Hendricks head toward a nearby farmhouse, while Megan and Parks search for the mythical meteorite. When they regroup and fortify the farmhouse, the mosquitoes begin to infiltrate again. Parks, a veteran like Hendricks, recounts his own war memories, and Hendricks confirms his past service as well, underscoring the sense that these creatures are an almost inevitable consequence of scientific meddling. The basement reveals a colossal nest of mosquito eggs, a chilling reminder of the scale of the threat. The group rigs the house to explode by severing a gas line and opening the windows, then climbs to the roof, taking turns in a dumbwaiter to reach safety.
Tragically, Hendricks’ turn comes too soon—the dumbwaiter fails and collapses into the basement, taking his life with it as the house erupts in flames. Earl and Parks are among those aboard the burning wreckage, but Parks survives the explosion by sheltering in a refrigerator, a small but uncanny act of luck that leaves him alive to tell the tale. As dawn breaks, Ray and Megan emerge to find Parks alive and breathing, having endured the catastrophe while hiding from the horrors behind the walls of the farmhouse. The film closes on a note of survival marred by loss, with a stark reminder of how a single, invisible threat can cascade into a nightmare that lingers long after sunrise.
