Detour

Two surveyors taking measurements in the Apalachicola National Forest in Leon County, Florida, are attacked by invisible creatures with glowing red eyes. Later that day, local resident Michael Asekov finds a bloodstained jacket with his son Lewis while hunting. Seeing an incomprehensible movement in the bushes, Asekov orders the boy to run home with the dog. As Lewis runs away, he hears gunshots.

Mulder and Scully, while in Florida, go with the narrow-minded and annoying FBI agents Kinzley and Stonesifer to a seminar on improving team spirit. Suddenly, the further road is blocked by the police. When Mulder gets out of the car, he is caught by the worried wife of Michael Asekov and informs him about her husband's disappearance and her son's story about what happened to them. Police officer Michelle Fazekas, who is also there, doubts the boy's testimony, but Mulder willingly agrees to help, if only not to go to a useless seminar in his opinion. He later explains to Scully that no animal in North America will attack a stronger prey when a weaker one is available.

At night, a dog suddenly starts barking in the Asekovs ' house. Mrs. Asekov lets her out, but when she tries to return to the house, she finds that the door is locked from the inside. Lewis, hearing his mother's screams and knocking from the street, gets out of bed, but sees a dark figure with glowing red eyes in the hallway. Lewis manages to escape, falling into the hands of the arriving Mulder. The next morning, Mulder, studying the tracks, discovers an unusual distribution of the creature's weight. Luring Mrs. Asekov outside also supports Mulder's hypothesis of the creature's paranormal nature.

The agents go into the woods with Michelle Fazekas and Jeff Glazer, a technician armed with a thermal imager. Glazer discovers two creatures that are invisible without the thermal imager, but they run in different directions. Divided into two groups, people try to chase them, but first Fazekas disappears, and then Glazer. The creature attacks Mulder and wounds him, although Scully manages to shoot the attacker. A wounded Mulder and Scully spend the night in the woods without food or water, trying unsuccessfully to build a fire.

The next morning, Scully falls into an underground tunnel containing the bodies of Fazekas, Asekov, and others. Scully notices red eyes in the depths of the cave and realizes that she is unarmed. Mulder drops the gun, but when he hears a noise in the bushes, he jumps down to his partner. Scully shoots into the darkness and kills a humanoid creature with skin like the bark of a tree. Next to the body, they find the words "Ad Noctum" (lat. "Into the dark"). The agents, as well as the survivors Fazekas and Asekova, are rescued by a search party led by Agents Kinzley and Stonesifer.

After leaving the forest, Mulder states that the creatures may have been evolved Spanish conquistadors who arrived here more than 400 years ago. Kinzley considers this a delusional idea, but Mulder develops the idea that centuries of alienation could lead to such drastic changes in the body. The agent comes to the conclusion that the creatures consider the appearance of humans on their territory a threat and resist it, which may threaten Scully's life. Mulder rushes to the motel to see Scully, but Scully calmly gathers her things. Mulder helps her with the suitcase, and the agents leave. After the door closes, red eyes flash under the bed in Scully's room.