S1E20 · Darkness Falls

In Olympic National Forest in Washington State, a group of loggers flee through the woods, trying to escape from an unseen force. They are eventually killed by a large swarm of small, glowing, green insects. Later, at FBI headquarters, Fox Mulder shows Dana Scully a photo of the missing loggers, telling her that another group of loggers disappeared in the forest in 1934. The two agents head to the forest, where they meet U.S. Forest Service employee Larry Moore and Steve Humphreys: head of security for the logging company. While driving through the forest, their truck hits caltrops left in the ground by eco-terrorists, forcing them to walk the rest of the way. Upon arriving at the camp site, Mulder and Scully find the cabins abandoned and the communication equipment destroyed. Searching the forest, they find a corpse encased in a large cocoon hanging from a tree.

While repairing one of the generators, Humphreys catches an eco-terrorist named Doug Spinney. He tells the group that there's a deadly swarm of insects in the forest and that they must avoid darkness to stay alive. The next morning, they find an old-growth tree cut down with an unexplained band of green contained within its growth rings. Spinney suspects that an organism that was dormant in the tree for centuries was disturbed when the tree was illegally cut down. Humphreys hikes down to Moore's truck but is killed by the swarm at nightfall. In the cabin, everyone else is kept safe by the light. The next morning, Spinney convinces Mulder to let him hike to his colleagues with gasoline so that he can return with a Jeep to pick them up. Scully and Moore confront Mulder, since this will leave them with little fuel for the generator.

The night passes with only a single lightbulb lighting the cabin, going out just as morning arrives. Mulder, Scully, and Moore hike down to the truck with a flat tire from camp, hoping to patch it, put on the spare, and escape. They find Humphreys's cocooned body. Spinney returns with the Jeep, telling the others that his friends are all dead. The Jeep hits another caltrop left in the ground, and Spinney is killed when he leaves the Jeep after dark. Moore and the agents are engulfed by the insects, which enter the vehicle through the air conditioning vents.

They are found soon after and brought to a quarantined facility in Winthrop, Washington, where one of the scientists tells Mulder that the forest is being bombarded with pesticides and controlled fire in the hopes of eradicating the insects. Mulder asks the scientist what will happen if the efforts fail, but is simply told "that is not an option."