Yozora's father was the boss of a certain yakuza crime syndicate.
After he was killed in a fight with a rival syndicate, a potential future leader by the name of Tatsumi has looked after her as a custodian / guardian.

With the syndicate on the brink of annihilation, Tatsumi takes Yozora away to live a normal life, and they begin a new life in a small room that is only about 10 meters square. (6 tatami mats)

Tatsumi utterly loves Yozora... Both like a father, and like an older brother.

But when exactly was it... that their family-like relationship, turned into something different.
To the relationship of a man, and a woman. Of "Male" and a "Female."

Day and night, the two indulge in one another's embrace. Drowning in intensely intimate sex...