"Syzygy" is the thirteenth episode of the third season of the science fiction television series The X-Files. The episode first aired in the United States on January 26, 1996, on Fox. It was written by series creator Chris Carter and directed by Rob Bowman, and featured guest appearances from Dana Wheeler-Nicholson, Lisa Robin Kelly, and Ryan Reynolds.

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, Mulder and Scully investigate the murders of high school students in a small town where everyone is acting strangely. They discover that two teens are responsible, due to a rare planetary alignment that affects their behavior.

The episode is a "monster-of-the-week" story, unconnected to the series' wider mythology. "Syzygy" earned a Nielsen household rating of 10.8, being watched by 16.04 million people in its initial broadcast. The episode received mixed reviews, with many critics and fans upset by the negative portrayal of Mulder and Scully.

The episode's title refers to an astronomical alignment of three celestial objects, usually the sun, the Earth, and a moon or planet. In addition, the episode contained various fan in-jokes, such as Scully getting upset because Mulder is always the driver, which was inspired by complaints from fans.

In Comity, New Hampshire, a group of high school students hold a eulogy for their dead friend, the purported victim of a local Satanic cult. Two girls, Terri Roberts (Lisa Robin Kelly) and Margi Kleinjan (Wendy Benson), get a ride home from a jock, Jay "Boom" DeBoom (Ryan Reynolds). The girls tell Boom that the cult seeks a blonde virgin as a next victim, convincing him to turn off the road. The next day, the police find Boom hanging from a cliff. Out of sight of the police, Terri and Margi sit at the top, laughing.

Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) arrive in Comity after arguing over directions along the way. They meet a local detective, Angela White (Dana Wheeler-Nicholson), and go to Boom's funeral. Scully, in a bad mood, is skeptical of these claims. The high school principal, Bob Spitz, interrupts the funeral by ranting about Satanic cults murdering their children when suddenly the coffin starts smoking and catches on fire. Mulder and Scully go into separate rooms to interview Margi and Terri, both of whom offer an identical story about a satanic ceremony where a baby was sacrificed. Scully thinks their stories are clichéd and points out that no account of a satanic conspiracy has ever been validated. Looking at the latest victim's body, Mulder and Detective White find a burn mark in the shape of a horned beast; Scully says she doesn't see anything. Mulder goes to see White to apologize for Scully's behavior and the two visit the local astrologist Madame Zirinka, who claims the town's crazy behavior is due to the rare planetary alignment of the planets Mars, Uranus, and Mercury. Terri and Margi watch basketball practice, lusting over one of the players, Scott, whose girlfriend is cheerleader Brenda (much to Terri and Margi's displeasure). When another of the players accidentally spills a table of drinks on them, they cause the basketball to bounce underneath the bleachers, which retract and slam shut on the boy when he goes to get the ball, killing him. Scully is angry at Mulder for ditching her to be with Detective White.

A town mob searches for a mass grave in the woods and finds a bag belonging to the town pediatrician filled with bones, which Spitz assumes belonged to a child. The angry mob goes to see the doctor, who claims that he had sold the bag to Terri Roberts. The bones turn out to belong to Terri's dog, Mr Tippy. Scully gets upset over a joke Mulder makes and tells him she's returning to Washington. Margi and Terri celebrate their birthday and Brenda uses a Ouija board to know who she will marry; everyone thinks it's Scott before the planchette veers away from the C and spells out Satan. Upset, Brenda rushes to the bathroom where Margi and Terri are chanting "Bloody Mary" and is locked in, then killed by glass from a shattered mirror. Detective White heads to Mulder's hotel room because she found a box which inside had her cat's collar, and then she attempts to seduce Mulder, but they are interrupted by Scully, who informs them about Brenda's death. Terri and Margi try to console Scott, who tells them off. Turned down, Terri is mad at him but Margi still likes him and leaves.

Mulder goes to visit Madame Zirinka again, who tells him that the planets come into alignment like this only once every 84 years, and additional alignments will cause anyone born on January 12, 1979 (Margi and Terri's birthdate) to have an abundance of cosmic energy. Margi goes to see Scott alone but an angry Terri arrives. The two argue with each other and end up accidentally killing Scott after causing one of the springs and the garage door to fly at him. Margi goes to Mulder, telling him that Terri is responsible for the murders, while Terri goes to see Scully and tells her the opposite. The agents call each other and bring both girls to the police station, where the place starts shaking, causing the furniture to move around and all the guns to go off on their own. Mulder locks the girls in a room together and their power goes away once the clock ticks midnight. When the town mob and Detective White finally see Terri and Margi as the culprits, Spitz claims it was the work of Satan. Mulder and Scully drive home, arguing again over directions; when Scully defiantly runs a stop sign, Mulder notifies her but she tells him to shut up, which he does.