The protagonist, Imose Hiroshi, had been overseas for five years because he fell out of favor with the company's slick manager. When he finally finishes his duty and returns home, he is greeted by his three beloved sisters. He decides to fully use the month-long vacation he has received from his company for his sisters, who have been waiting for him... Meeting them after five years, they have grown up to be attractive girls with abundant fruits. What's more, all of them had developed an increased love for their brother that they had harbored since early childhood! On the day the National Diet passed the "special law allowing you to marry your younger sister," nicknamed "Sister Zone" after great controversy as a special remedy for the declining birthrates with no improvement in sight, the sisters furiously started making advances, literally putting their life on the line! What will Hiroshi do in this situation? [Translated from Official Website]