Fate/Zero Second Season

Caster has summoned a gigantic Horror. If it is not eliminated before it finds a new source of mana, the Grail War might have to be called off, to say nothing of Fuyuki and the entire world being put in danger. Rider appeals to Saber and Lancer, and the three Heroic Spirits decide to fight side by side.

The Heroic Spirits battle the evil Horror summoned by Caster. Elsewhere, there is another man who is putting his life on the line. Kariya Matou directs his hatred for magic, which has caused so much heartbreak for Aoi Tohsaka and Sakura Matou, at Tokiomi Tohsaka. Someone is watching as both face off against their opponent and live up to their convictions...

Kayneth succeeds in obtaining a Command Seal for exterminating Caster. But just as he thinks this will allow him to re-enter the Grail War, he learns that someone has taken Sola-Ui captive, and he berates Lancer harshly for allowing it to happen. He swears his loyalty to his Master, and Lancer despairs that the more he tries to live up to the ideals of chivalry, the wider the gap becomes.

The Grail War progresses as fighters are knocked out of the running one by one. The building tension causes the remaining fighters to reevaluate their motives. Also, someone leaves a last request with their family... Someone plots to meet in secret. And in the midst of all this, Saber's team receives a request from an unexpected source.

Kerry is a young boy living on a small island belonging to a southern country. His father stays cooped up in their house, conducting mysterious research, and a girl named Shirley that he cares deeply for assists his father. Shirley asks the boy a question. "Kerry, what kind of man do you want to grow up to be?" This is the story of the origins of a boy named Kiritsugu Emiya, AKA "Kerry".

The justice that Kiritsugu chose has taken something precious away from him, as if to balance the scales. Kiritsugu Emiya, who has chosen to leave the island and live with Natalia as a hunter, becomes embroiled in a wide variety of battles. Even as he realizes the cruel nature of his "justice" during these battles, Kiritsugu sets out towards a new battle with Natalia.

The Holy Grail War is building to a climax. This begins to have an effect on the body of Irisviel, the Grail's guardian. Kiritsugu leaves Irisviel's side, the coldly ruthless expression he once wore while on the battlefield with Maiya Hisau back on his face. Elsewhere, Waver goes to a certain place to aid Rider, who expended most of his mana during the battle with Caster.

Saber swears to a dying Maiya that she will retrieve Irisviel, whatever the cost. Saber races down the road on her V-MAX in pursuit of Rider, who has made off with Irisviel. Meanwhile, Kiritsugu has misgivings about this situation and goes after Iri from a different tack. In the process, he learns the truth about something.

Still unable to locate Irisviel, Saber searches high and low through Fuyuki with a haggard expession on her face. However, Kiritsugu ignores Saber completely. Elsewhere, Kirei, who has found a reason to fight, awaits Kiritsugu's arrival. Dawn is about to rise on the final day of the fourth Holy Grail War. Sensing this, Waver also steels his resolve once again.

Taking the signal flare that Kirei fired as a signal, the resolute teams gather one after another. Rider had raced to the battefield with Waver, but Archer blocks his path. The King of Heroes and the King of Conquerors. The battle between kings has begun. At this same time, Waver is also forced into a fateful confrontation.

Having learned Berserker's true identity and the hatred directed towards her, Saber is battered by despair. Losing the will to fight, all she can do is block the blows of the pitch-black sword that drips with malice. Elsewhere, Kirei and Kiritsugu finally confront each other directly. They struggle fiercely to see who will live and who will die, neither one yielding a single step.

The story reaches the beginning (zero) as the 4th Holy Grail War comes to an end…