Mulder and Scully have to investigate a bizarre case, one which involves a number of invisible animals!

An unseen force suddenly breaks a pane of glass and causes havoc through the streets eventually killing a workman nearby. Soon after, 43 miles from the zoo where it came from, an elephant appears out of nowhere and falls to the floor dying.

Mulder has his own theory on what happened, he believes that an invisible elephant did the damage. Later, the agents go to see Ed Meecham, the chief of operations at the zoo, Willa Ambrose and Kyle Lang, a member of the Wild Again Organization dedicated to liberating animals locked away in zoos.

With these interviews not getting them very far, Mulder decides to teleconference the Lone Gunmen who explain that the zoo is actually close to a UFO hotspot and that no animal in the zoo has ever been pregnant. Meanwhile, Scully pursues her own avenue with the WAO and follows one of their members into the zoo (she believes they are responsible). He climbs a fence but is soon mauled to death by an unseen force and a cage which contained a tiger, now doesn't.

Mulder soon suggests that they have a talk with Sophie, Ambrose's gorilla who is able to speak fluent sign language. She repeats the same cryptic words over and over again though, "Light, afraid". Mulder has his own explanation for these words. Mulder realizes that the animals must have been pregnant at one time, just not by conventional means and organizes an autopsy on the dead elephant which turns out to have actually been pregnant at one time although she had never mated in her life.

Later, the same tiger who escaped is tracked down and killed by Meecham. The tiger was also pregnant. Because of these incidents the zoo is closed down and funding is shut off.

Mulder finally explains his theory to Scully and thinks that these animals are being abducted, inseminated and returned miles away from the zoo.

Later, when Sophie is taken into custody, Ambrose goes to Kyle for help but he refuses only to change his mind and turn up outside Sophie's cage where he is crushed to death by a mound of falling crates. Mulder finds out that Meecham was paid by Ambrose to kidnap Sophie and so stalks him to a warehouse. Mulder finds the ape but is trapped in the same room as it and is charged by it only to watch it saying "Man Save Man" before being abducted in a flash of light.

When Scully finds Mulder they both rush out to Sophie who is found dead after being hit by a car. Ambrose and Meecham are both charged with killing Kyle but Mulder is still concerned about the message which Sophie left with him and explains that "in the simple words of a creature whose own future is uncertain, will man save man?". Nitpicking