"Fate/Zero" is a prequel to the "Fate/stay night" series, set ten years prior to the events of the original story. The narrative revolves around the Fourth Holy Grail War, a secret conflict in which seven mages, known as Masters, summon heroic spirits, or Servants, to compete for the Holy Grail, a mythical artifact capable of granting any wish. The war takes place in Fuyuki City, where the Masters and their Servants engage in strategic battles, alliances, and betrayals to eliminate their rivals and claim the Grail.

The story begins with the summoning of the Servants, each representing a different class: Saber, Archer, Lancer, Rider, Berserker, Assassin, and Caster. Each Servant is a legendary figure from history or mythology, bound to their Master by a magical contract. The Masters include Kiritsugu Emiya, a pragmatic and morally ambiguous mercenary who summons Saber, the spirit of King Arthur; Kirei Kotomine, a conflicted priest aligned with the Church who commands Assassin; and Tokiomi Tohsaka, a traditional mage who summons Archer, the arrogant and powerful Gilgamesh. Other participants include Waver Velvet, a young and inexperienced mage who summons Rider, the boisterous Alexander the Great; Kayneth El-Melloi Archibald, a prideful mage who partners with Lancer, the honorable Diarmuid Ua Duibhne; and Kariya Matou, a desperate man seeking to save his family by summoning Berserker, a corrupted and uncontrollable spirit.

The narrative explores the motivations and ideologies of the Masters and Servants, delving into themes of ambition, morality, and the cost of power. Kiritsugu Emiya, the central protagonist, operates with a utilitarian philosophy, willing to sacrifice individuals for the greater good, which often clashes with Saber's chivalric ideals. Kirei Kotomine, initially portrayed as a dutiful priest, grapples with his inner emptiness and growing fascination with destruction, leading to his eventual alliance with Gilgamesh. The interactions between Masters and Servants reveal complex dynamics, such as the mutual respect between Waver and Rider, and the tragic bond between Kariya and Berserker.

As the war progresses, alliances shift, and the true nature of the Holy Grail is gradually revealed. The Grail, far from being a benevolent wish-granting device, is corrupted and seeks to manifest its destructive potential through the conflict. The final battles culminate in a chaotic and devastating climax, with Kiritsugu forced to confront the Grail's malevolent influence and make a choice that will shape the future of Fuyuki City. The series concludes with the aftermath of the war, setting the stage for the events of "Fate/stay night."

"Fate/Zero" is characterized by its intricate plot, morally complex characters, and exploration of philosophical themes. The interplay between the Masters and Servants, as well as the gradual unraveling of the Grail's true nature, creates a compelling narrative that balances action, strategy, and introspection. The series serves as both a standalone story and a foundation for the broader "Fate" universe, providing context and depth to the events of its successor.