Oikawa is the emperor of a country
he doesn't have any heirs, so he's decided to find a worthy child and raise them
on one of his country wide orphanage visits he meets Kageyama
he stands out from the other kids, so Oikawa takes him aside, asks a few questions, decides to adopt him
the rest of Oikawa's close friends (who double as Kageyama's tutors) take a liking to Kageyama because he's smart/talented
Mattsun/Makki like teasing him
when Kageyama comes of age, Oikawa slowly starts getting more paranoid
he's afraid that Kageyama might turn his kingdom against him
he banishes Kageyama to the furthest corner of the kingdom under the excuse of sending him to "enlighten his mind"
on the way he runs into Noya/Tanaka (literally)
they take him to Daichi, who's the leader of their mercenary group
(they're camping at the place in the forest where Kageyama was supposed to go)
so Kageyama joins the self-proclaimed Karasuno traveling mercenaries
travels around, gets paid for his work, trains, makes friends
eventually they start to hear about unrest and dissatisfaction with the king and the way he rules
there's talk of rebellion
the rest of Karasuno eventually find out who Kageyama really is
so they travel to the capital
Oikawa knows they're coming
he thinks Kageyama will be easy to defeat
he doesn't know that Kageyama's changed from before
INSERT FIGHTING, friends attempting to die for each other but that doesn't happen don't worry
once they near the castle, Oikawa sends out his personal guards to get rid of Kageyama + the rest of Karasuno
except both sides are kinda reluctant to fight each other
Makki and Mattsun actually pretend to surrender bc they + Iwaizumi all think that Oikawa's being ridiculous
everyone else is furious (kind of) because it almost feels like they're betraying their king
Iwaizumi gives the group vague directions into the castle to where Oikawa is
they get to the throne room
Oikawa insists that only Kageyama enter the room
insert long speech here where he tries to make Kageyama feel bad
and it gets up to the point where he has like his sword against Kageyama's neck
but then Hinata's standing in the doorway with his bow all nocked and ready to fire
Iwaizumi also comes in, tries to knock some sense into Oikawa
Oikawa eventually surrenders, offers the throne to Kageyama
Kageyama refuses to take his place, says Oikawa is more fit to rule than he is
he ends up going back and traveling with Karasuno