Our protagonist is close to graduating from university.
At that time, his parents are transferred overseas, leaving him alone with his younger sister.
Unable to leave his cute little sister alone, he returns home,
but he finds that she become a NEET and has developed a severe case of chuunibyou!

Should the protagonist continue slacking off, or should he head to his computer and aim to become a creator?
Or should he try to reform his sister, who has developed a self-proclaimed multiple personality disorder?
But how...?
The one who teaches him the solution is one of his sister's self-proclaimed multiple personalities.
In fact, she has loved him for a long time...
If he showers her with that love, she will definitely reform! And if they get married, that's for sure!
...Um...What kind of erotic game is that?
His sister's words are full of things to criticize, but if he goes along with it, maybe...
This is the glimmer of hope for our protagonist.
And above all, he himself loves his sister.
If that's the case, then perhaps this is his chance after all!

Thus began the life of the protagonist and his chuunibyou, NEET sister.