Six months after the events of Fate/stay night, Mage's Association member Bazett Fraga McRemitz wakes on the fourth day of a Holy Grail War she has no memory of starting, bonded to a new Servant called Avenger. Across town, Shirou Emiya's quiet postwar life is disrupted by mysterious occurrences that shouldn't be possible with the Grail War already over. As the two storylines converge, Bazett and Shirou discover they're both trapped in the same four-day loop, repeating without end until they find what's causing it.