Mai, who has a perfect brother-in-law, said, "I'm not related by blood, so I will definitely marry my brother!" I have dreamed of marrying my brother-in-law since I was a child.
However, one day, my brother-in-law is a character in a death game manga, and a year later, he holds a death game involving his classmates, and remembers killing his sister Mai in the process.
Mai plans a variety of surprises for her brother-in-law every day from the line her brother-in-law said in the manga, "Everything is as expected and boring", but every day, her brother-in-law begins to show bizarre signs...