The plot so far:
Yamatai was a perfect nation under the rulership of God-Queen Himiko, aided by the power of her prophecy.
... however, this only lasted until she and her successor Iyo died.
The country was overly reliant on the existence of a perfect God-Queen to direct it.
In the absence of such, it began to fall apart.
Thereafter, the grudge of the Country of Kuna, once vanquished by the Yamatai, was somehow sufficient to summon forth the man named Serizawa Kamo.
It's implied that said Grudge was able to acquire something akin to a Grail.
Serizawa was a man descended of upper-class Kyoto samurai.
In the Greater History, he was a Captain of the Shinsengumi recorded to have abused the authority vested in the organization.
He regarded himself as a visionary with the strength to shoulder the august task of saving the country — unalike to the subordinates of other Shinsengumi Captain, Kondo Isami.
This was regarded as empty egotism by the Kondo faction.
After he publicly incited a series of fights, a division of the Kondo faction — led by Hijikata — assassinated him.
Members of the group that took part the act included Okita, Saitou, and Yamanami.
Summoned into the past, Serizawa regarded his circumstances as a ripe opportunity to perform the duties ascribed to him by fate — to tear down the Yamatai and forge a more enlightened nation in its place.
Yamatai was in his opinion a nation wrought with flaws from the start, as it couldn't subsist in the absence of its human pillar.
Thus, to proceed, he planned to rouse 'the Gods of Antiquity' — monsters by which to purge the remnants of the Yamatai.
The particular ritual he hoped to employ as to achieve this end required a sacrifice — a Yorishiro (依り代) as to contain and anchor the Grudge of the Kuna, otherwise manifest as a black mist referred to as Magatsu-Yami.
Ergo, he needed a vessel of sufficient quality to withstand the Grudge, and so summoned Maou Nobunaga.
Begged by the spiritual remnants of the God-Queen Iyo to buy time until Himiko manifested, Nobunaga resisted Serizawa's manipulations — somehow summoning members of Chaldea into the past.
The summoned Servants were envesseled in Chibi-Nobu Haniwa clay figurines, and positioned as to guard the shrines surrounding the village of the Yamatai.
The shrines themselves were somehow repurposed as to anchor the Magatsu-Yami across a large stretch of land, rather than permitting it to simply gather within Nobunaga's Saint Graph.
As of the beginning of the event, Guda and Himiko, summoned forth to 3rd century Yamatai, come to encounter Saitou and Yamanami, who initially position themselves as allies.
Being that the shrines and the Haniwa guarding them appear to exude Magatsu-Yami — a corruptive force that drives humans to insanity, and obstructs the raising of crops — Saitou and Yamanami are able to easily manipulate Guda's party into assisting them in purging the Haniwa.
In reality, though, they were acting as subordinates to Serizawa, sent to manipulate Chaldea into doing the dirty work of clearing the Magatsu-Yami anchors set up by Nobunaga.
Serizawa Kamo is an arrogant elitist with an exaggerated opinion of his own worth — even as he happens to be competent enough to act as the antagonist behind this event. (That is, he's more or less able to back up his arrogance with legitimate ability, but he's still an asshole.)
Comparatively, Kondo Isami was somebody who — while apparently being of a more upstanding character — literally didn't achieve anything notable as of the Greater History.
Under his leadership, the Shinsengumi didn't save Japan, and weren't able to obtain anything aside from their own destruction.
Yamanami Keisuke feels that this outcome came about because he sided with Kondo faction in assisting with Serizawa's assassination.
Even before his death, he began to think that maybe Serizawa for all his faults could've served as a "necessary evil."
As to Saitou Hajime — he's somebody who witnessed the outcome of Kondo's leadership; the utter annihilation of the Shinsengumi.
Also, he seems to be friends with Yamanami — so, his actions in the event so far seem to be something along the lines of "maybe i'll go along with Yamanami's judgment."
But, it's hard to tell what he's thinking at present time.
On confrontation, Serizawa sets Nobu-controlled-by-the-Magatsu-Yami against Guda's party, and escapes, giving on the way out that it doesn't particularly matter as who emerges alive, as Nobu was intended to be used as a human sacrifice anyhow.
After you defeat Nobu, she regains control of her body — but tells Guda's party not to approach.
It's explained that as Serizawa stated, Nobu will die irrelevant of whatever Guda's party does.
Again, all of the business with the Chibi-Nobu was just something to buy sufficient time for Himiko to manifest within the era.
Nobukatsu takes issue with Nobunaga's resignation to her fate, asserting that she can't die.
He asks Himiko to save his sister, but hers aren't the sort of power that would permit for such a thing.
Desperate, he asks Guda to do something, as she's Nobu's Master.
At this point, Nobu tells him to shut up.
She says she was always annoyed that he chased her around when they were kids; that he's overly clingy; that in fact, she never wants to see him appear before her again.
Nobu: "Hereforth shall come a world absent of Oda Nobunaga. Therefore, walk by your own two feet, and forge a path of your own."
Before dying, Nobu asks Guda to take care of her brother for her.
Incidentally, the first part of the event's development quest is literally that, with just his skills alone, Nobukatsu brought Yamatai back from the brink of destruction.
However, in the event shop and in storyline, he tells Guda, "This? This is nothing. just child's play. If my sister were handling this, we'd already be up to the industrial revolution."
In essence, he fails to recognize that he himself is of any worth at all.
