"Fate/Grand Order: First Order" is a 72-minute anime special that serves as an adaptation of the prologue chapter of the mobile game "Fate/Grand Order." The story is set in a world where humanity's survival is threatened by the incineration of human history. Chaldea Security Organization, a secretive institution tasked with preserving the future of humanity, discovers that the extinction of mankind is imminent. Through its advanced observation system, Chaldea identifies the cause as singularities—distortions in time that disrupt the proper flow of history. To prevent this catastrophe, Chaldea deploys Masters, individuals capable of summoning and commanding Heroic Spirits, to resolve these anomalies.

The protagonist, Ritsuka Fujimaru, is a novice Master who, alongside their partner Mash Kyrielight, is sent to the first singularity: Fuyuki City, a version of the city engulfed in flames and overrun by hostile Servants. Mash, a Demi-Servant, wields the powers of the Shielder class and serves as Ritsuka's protector. The duo encounters Cú Chulainn, a Lancer-class Servant, who initially opposes them but later aids in their mission. They also face Saber Alter, a corrupted version of King Arthur, who serves as the singularity's primary antagonist.

As Ritsuka and Mash navigate the burning city, they uncover the singularity's core, a Holy Grail that sustains the distortion. The Grail's presence has twisted the timeline, creating a chaotic and destructive version of Fuyuki. The narrative focuses on the duo's efforts to confront Saber Alter and retrieve the Grail, which is essential to restoring the timeline. The story emphasizes the partnership between Ritsuka and Mash, highlighting Mash's growth as a Servant and her determination to protect humanity.

The anime special concludes with Ritsuka and Mash successfully defeating Saber Alter and securing the Grail, thereby resolving the Fuyuki singularity. This victory marks the beginning of their journey as Masters tasked with addressing future singularities to safeguard human history. The story sets the stage for the larger narrative of "Fate/Grand Order," introducing key concepts such as the summoning of Heroic Spirits, the role of Masters, and the existential threat posed by the incineration of history. The characters' development and the stakes of their mission are central to the plot, providing a foundation for the expansive lore of the franchise.