"All Souls" is the seventeenth episode of the fifth season of the American science fiction television series The X-Files. The episode originally aired on the Fox network on April 26, 1998. The episode's teleplay was written by Frank Spotnitz and John Shiban, from a story by Dan Angel and Billy Brown; it was directed by Allen Coulter. The episode is a "Monster-of-the-Week" story, a stand-alone plot which is unconnected to the mythology, or overarching fictional history, of The X-Files. "All Souls" received a Nielsen household rating of 8.5 and was watched by 13.44 million viewers in its initial broadcast. It received mixed reviews from television critics.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files. In this episode, the unexplained death of mentally and physically disabled girl prompts Father McCue (Arnie Walters) to ask Scully for her help, but her investigation leads her to a mystery involving Nephilim—children of mortal women and angels. Scully soon learns that Aaron Starkey (Glenn Morshower), a department of social services worker and demon in disguise, is after the girls, in order that the Devil may control their power.

The original version of "All Souls" was a simple story about Mulder, Scully, and angels. Shiban and Spotnitz, however, overhauled the idea and added elements extrapolated from the season's earlier "Christmas Carol" and "Emily" two-parter, making "All Souls" the "unofficial third part" of its story arc. The entry also contained several elaborate effects, which were achieved via makeup and CGI. After they viewed the final cut of the installment, Shiban and Spotnitz decided to frame the action around Scully confessing her story to a priest in a confessional.

In Alexandria, Virginia, sixteen-year-old Dara Kernof (Emily Perkins), a mentally and physically disabled girl who uses a wheelchair, somehow manages to leave her house in the middle of the night, soon after her baptism. Her father, Lance (Eric Keenleyside), eventually finds her outside with her arms raised upwards towards a strange figure. Suddenly, lightning flashes and the figure disappears. When Lance reaches Dara, he realizes she is dead and her eyes have been burned out. Eventually, Dana Scully (Gillian Anderson) is contacted by Father McCue (Arnie Walters), who asks if she would be willing to assist the family in figuring out what exactly happened. Soon thereafter, Scully visits the Kernofs and learns that Dara was adopted. Given her severe spinal deformities, Scully is unable to explain how Dara walked, let alone got outside. Lance then tells Scully that he saw a strange figure before her; he confides in her that he thinks the mysterious being was the Devil.

While this is going on, a priest named Father Gregory (Jody Racicot) visits a hospital to visit Dara's twin, Paula Koklos (Perkins), but he is stopped by a social worker named Aaron Starkey (Glenn Morshower). That night, Paula dies mysteriously when a man enters her room. Fox Mulder (David Duchovny) soon learns that Paula and Dara were not twins, but rather two of four quadruplets. The agents also learn that Father Gregory was hoping to adopt Paula and they visit the priest. Gregory protests that he is innocent and was simply trying to protect Paula by taking her under his wing. Later, Scully experiences a vision of her daughter Emily (Lauren Diewold).

Mulder soon learns the identity of another sister (Perkins), who is troubled and homeless. With Starkey's help, Mulder finds the girl in a rundown part of the city, but the figure from the beginning of the episode kills her before Mulder can get to her. Mulder discovers Father Gregory at the scene and believes that he is the perpetrator of the crimes. Under questioning by Mulder and Scully, Father Gregory insists that he was attempting to ward off the Devil, who was hoping to claim the girls' souls. He is adamant that the fourth and final sister must be located. Later, when the agents have left the room, Starkey enters and asks for the whereabouts of this last girl. Gregory remains silent and soon begins to burn; it is revealed that Starkey is the Devil.

That night, the figure from the beginning of the episode presents itself to Scully, revealing that it is a Seraph. This seraph had fathered four nephilim (i.e. the disabled quadruplets) and God sent the angel to earth to return the girls to Heaven. The angel is attempting to outmaneuver the Devil, who hopes to use the girls' souls for his own nefarious plots. Scully and Starkey later find the fourth girl, Roberta Dyer (Perkins) at Gregory's church. However, Scully sees that Starkey has a horned shadow, revealing his true origins. As Scully tries to help Roberta escape, the women are confronted by the Seraph. Scully reluctantly lets go of Roberta's hand, after seeing Emily in place of Roberta, and lets Roberta enter Heaven.

The script for "All Souls" was written by John Shiban and Frank Spotnitz, and directed by Allen Coulter. The original genesis for the episode was an idea developed by Dan Angel and Billy Brown, two story board editors for the series that had left after Christmas of 1997. In this incarnation, the plot was radically different; according to Shiban, the story was "about Mulder and Scully and angels, but it never quite worked the way it was originally conceived." Shiban and Spotnitz decided to overhaul the script and feature elements of the earlier "Christmas Carol" and "Emily" story arc, as the writers wanted to include the "very universal" idea of Scully exploring feelings for her deceased daughter. This episode, according to Andy Meisler, thus became the "unofficial third part" in the story. With this episode, Scully dons the role of the believer, whereas Mulder plays the skeptic. This "Mulder-Scully criss-cross" accentuated a major theme for the fifth season.