It is the year 2096.

Almost a century has passed since magic was established as actual technology.
A pair of siblings were about to enter the winter season of their second year in high school.
Tatsuya, the older brother, bears a fatal flaw in his magic abilities.

Meanwhile, the beautiful, intelligent, and perfect younger sister, Miyuki, has unmatched magical talent.
An irregular and a star student. Despite their differences, the pair of siblings lived together in harmony.
That is, until they receive a certain letter...

The letter contained an invitation to the New Year's Banquet, to be hosted at the Yotsuba Clan's head manor.
The Head of the Yotsuba Clan, Maya Yotsuba, and the heads of each branch family will be in attendance...
There, the next Head of the Yotsuba Clan will be announced.

Miyuki dreaded the prospect of being designated as the next Head of the Clan.
For if she were to assume that mantle, she would no longer be permitted to remain single; she would be required to marry. And that, in turn, would mean having to let go of the days she spent by Tatsuya's side.

However, Miyuki becoming the Head of the Clan would allow her to transform the status of Tatsuya—who, as a Guardian, has long been treated with disdain. Hoping to provide Tatsuya's freedom, Miyuki makes a momentous decision while en route to the Yotsuba manor.

"Brother... I will become the Head of the Yotsuba Clan."

What fate awaits the "Supreme Siblings"?
And what shocking truth lies hidden between them?

Amidst the swirling conspiracies of the Yotsuba clan, the siblings' story draws toward its conclusion.