After the events of the season six finale, Walter Skinner and Michael Kritschgau are desperately attempting to find the truth behind the so-called alien object. Meanwhile, Fox Mulder is still imprisoned by his own frenetic brain activity. Dana Scully and Skinner are unaware of FBI Special Agent Diana Fowley's duplicity—she is working for The Smoking Man. Scully then travels to Africa to unravel the secrets of the alien artifacts, finding something that looks like a spaceship buried under the shoreline off the Côte d'Ivoire coast. The object may prove that life originated elsewhere, and all religion is based on the Navajo contact with alien life. Unsuccessful, Scully returns from Africa to revisit Mulder in Washington, D.C., but instead she finds out that he has disappeared. She contacts Kritschgau and Skinner to find her partner. The Smoking Man has taken Mulder to a place where all his problems seem to have disappeared. Fowley helps Scully locate Mulder, which leads to her death at the hands of The Smoking Man.

While investigating a bizarre disappearance of a young girl from her home, Mulder becomes obsessed with the number of children who have vanished in similar circumstances. Scully fears that he is emotionally involved due to his sister's disappearance. At the same time it is revealed to him that his mother, Teena Mulder, committed suicide. He then tries to prove that his mother did not take her own life, but is ultimately forced to accept that his mother's death was by her own hand. He is led by a man whose son disappeared years earlier to another truth—that his sister may be among the souls taken by "walk-ins", saving the souls of children doomed to live unhappy lives. Together they locate evidence that proves that Samantha was abducted by The Smoking Man, and was forced to live in a now-abandoned US Army base. It is later revealed that Samantha had become a "walk-in" spirit.