The protagonist is born into a distinguished family with 4 older sisters. When they are young, the siblings are always together and even promise to get married. However, as they grow older, the distance between them also widens.

Then one day, their grandfather suddenly tells the sisters that they must become brides within the year. However, they are forbidden to associate with the males and ordered to protect their virginities until then.

But the sisters are still growing girls and therefore, very interested in sexual matters. They want to have at least a passionate experience first before marrying. Thus, they turn to the protagonist and pretty much force him to cooperate.

[Translated from Official Site]