Invisible animals are rampaging through town, destroying property and trampling a federal employee. On arriving in town, the agents begin their search at a nearby zoo with the distinction of never being the site of live birth. Whether that's because as Mulder suspects, Aliens are 'harvesting'' the young or, as Scully insists, an animal rights group determination to close down the zoo, the only reliable witness appears to be a hand-signing gorilla named Sophie.
