Fujimaru and company are dumbfounded by Gilgamesh's sudden visitation of the Chaldean Embassy. There, they are given a new request from the king which is to investigate the Persian Gulf. Merlin and Ana cannot join as they have no travel permits. In other words, this will be Fujimaru's first journey alone with Mash in a while. But there sat the noble king, reciting a poem and cocking his head in contemplation. Having fun with tales of the past, Fujimaru and company's journey will finally become a lively one.

Fujimaru and entourage, in order to rescue the people left behind in the fortress city of Nippur after communications were severed, begin their battle against the fake Enkidu who had been awaiting them. With Fou's help, they were somehow managing to keep things even, but in the midst of battle, the Goddess of the Demonic Beasts finally appears. Even Merlin is at his wit's end facing her overwhelming power. Meanwhile, Ushiwakamaru and Leonidas I engage in a spectacular battle, standing up defiant against the Goddess of the Demonic Beasts. In order to break through the situation where it is hard to deliver a wounding blow, the two Servants unleash their Noble Phantasms with desperate resolve.

Fujimaru and company greet Ishtar to their ranks without hitches, and gain information from Gilgamesh about the key for defeating Gorgon, "the Axe of Marduk" stowed away in Eridu within the southern jungle.

Fujimaru and company ever so easily have Jaguar Warrior join their ranks, and are reunited with the soldiers of Uruk, thought to be all dead, outside Eridu.

The power of the Grail, now in the hands of the Laḫmu, has transformed the Persian Gulf into the Chaos Tide. Any living thing that touches this sea is instantly mutated into a Demonic Beast.

As the Chaos Tide advances toward Uruk, Fujimaru and company form a desperate plan to destroy Tiamat as a last-ditch effort to save the city.

Tiamat falls into the underworld, but there, she transforms into one of the Evils of Humanity: Beast II. Hassan-i Sabbah, the first Old Man of the Mountain, intervenes, using his power as Grand Assassin to deprive Beast II of her immortality.