In Washington, D.C., an artist's model is brutally slashed to death after he finishes a session posing for a group of art students. The F.B.I. links one of the students, John Mostow, to the model's death, and to several other killings of young males over a three year period. Agents Patterson and Nemhauser arrest an unruly Mostow in his studio apartment. During the struggle, Mostow bites Nemhauser on the arm. Five days after the arrest, the body of another young male victim, bearing the same facial mutilation, is found six miles away.

Mulder and Scully interview Mostow in his jail cell. He claims he was possessed by a dark spirit, an entity that forced him to kill. Afterward, the agents meet with Patterson, who worked on solving the serial killings for three years. In private, Mulder tells Scully that he and Patterson have held a grudge ever since his days as a student at the bureau. Patterson was well-respected and even envied by many students, but Mulder failed to step in line. He didn't believe in Patterson's teachings that the only way to find a monster is to become one yourself.

The agents visit Mostow's studio, where they find sketch after sketch of gargoyles covering the room. Mostow believed the images protected him from the demon. Mulder and Scully discover more bodies-covered with clay-hidden within a secret room. Shortly thereafter, a man employed as a glassblower is viciously attacked by a figure armed with a carpet knife.

Mulder visits a library and reads literature pertaining to gargoyles. He then returns to Mostow's apartment, where he draws and sculpts images of the creatures. A dark figure emerges from the shadows. Mulder chases the figure to an industrial area. The creature attacks the agent, but during the struggle, Mulder falls from a gangway. The figure runs off into the darkness. Later, Mulder tells Scully that the creature exists. Scully is skeptical, and as she leaves the scene, she discovers broken pieces of a knife. The crime lab finds Mulder's finger print on the blade. Scully pays a visit to the F.B.I's evidence room, but Mostow's confiscated knife, seized by agents, is no where to be found.

Mulder returns to Mostow's studio, where he discovers Nemhauser's dead body encased in clay. When Patterson arrives at the studio, Mulder accuses him of killing Nemhauser and everyone else murdered since Mostow's incarceration. Mulder describes how Patterson became so obsessed with finding the killer, imaging everything the killer imagined, sinking himself so deeply into the case that finally, he became the very thing he so desperately sought to catch. Scully arrives at the studio, distracting Mulder. Patterson runs off and makes his way onto a rooftop. A struggle ensues, during which Patterson's face transforms into a hideous gargoyle. Mulder manages to grab his gun. He shoots Patterson, wounding him. Patterson is taken to jail.