Mulder and Scully investigate the disappearance of thirty loggers who were clearing trees in a remote forest in Washington State. They join forces with Larry Moore, a member of the Federal Forest Service, and Steve Humphreys, an employee of the lumber company. Moore and Humphreys suspect that the loggers may have been victimized by militant environmentalists-or eccoterrorists-who have waged a campaign of destruction against lumber companies in retaliation for ruining American forests.
     As the group travels along a dirt road, a tire on their 4 x 4 falls victim to sabotage. Left with little choice, the foursome set out on foot. They discover a deserted logger's encampment, and hanging from a nearby tree, a strange cocoon-like object. When the cocoon is cut open, the group finds a grotesquely shrunken corpse inside. Shortly thereafter, Doug Spinney, an eccoterrorist suspected of sabotaging equipment, arrives at the camp on foot, his jeep having run out of gasoline. He warns that strange, insect-like creatures inhabit the forest, and when darkness falls they descend from the sky in search of prey. The only thing the creatures fear, Spinney adds, is light.
     Humphreys scoffs at Spinney's story, believing it to be a cover for the loggers' disappearance. Frustrated by Moore's refusal to arrest Spinney, Humphreys begins to hike back to town alone. Along the way, the tiny insects engulf him.
     Mulder, Scully, Moore and Spinney examine a giant tree trunk lying in the woods. Moore takes a core sample, and finds tiny insects feeding in its center. This leads to speculation that the once dormant creatures were released when the loggers cut down the tree.
     Mulder allows Spinney to transport the remaining gasoline used to fuel the cabin's generator to the stalled jeep-which is two valleys away. Moore chastises Mulder's decision, certain the creatures will attack once darkness falls and the generator runs out of fuel. Scully panics when millions of the phosphorescent bugs swarm inside the cabin, waiting for the last glow of a lightbulb to fade.
     But the fuel in the generator lasts until sunrise, keeping one light glowing and sparing the group a grisly fate. Realizing they will be attacked come nightfall, the threesome decide to carry a good tire from one of the sabotaged vehicles to the abandoned 4 x 4. But when they locate the 4 x 4, they are horrified to discover Humphrey's cocooned corpse inside.
     Spinney suddenly appears with the jeep as he had promised. He transports the others through the forest at a high rate of speed, hoping to outrun the darkness. Along the way, the jeep's tires are punctured by a sharp metal device Spinney himself had planted on the dirt road to sabotage the logger's vehicles. A swarm of insects descends upon the jeep and everyone inside is cocooned. The group is rescued by military medical personnel, then transported to a hospital for treatment.