In 2016 AD, humanity's foundation is burned away by the Mage King Solomon. Chaldea, a secret organization of mages tasked with safeguarding mankind's future, detected humanity's extinction in 2015. This led to an operation to fix the Singularities in history created by Holy Grails scattered across time and space—Operation Grand Order.
Using Rayshift time travel technology, Chaldea's final master, Ritsuka Fujimaru, along with his demi-servant Mash Kyrielight, has journeyed through and resolved six Singularities. Now, they set out for their most perilous mission yet: a civilization from the Age of Gods, Mesopotamia in 2655 BC. Upon arrival, Ritsuka and Mash find the land overrun by Demonic Beasts that attack people and settlements. Amid the chaos and fear stands humanity's last bastion—Uruk, a fortified city serving as the front line in the battle against these beasts. The defensive effort is led by none other than King Gilgamesh, the King of Heroes, who summoned Heroic Spirits and took up the role of a mage to protect his city.
Together with Gilgamesh and the summoned servants, Ritsuka and Mash must defend Uruk from the magical beasts' assault and defeat the Three Goddess Alliance, which seeks to eradicate all humans. Meanwhile, an even greater threat waits beneath Uruk, stirring toward awakening.