“We are step-siblings, so we can get married and do all kinds

of naughty stuff you know?”

The protagonist has had a stepsister, Yumi Ichiiri, one year younger than him, since a year ago. 

Although he referred to her as his stepsister, he had always treated her as his little sister... or so he thought. 

One day, after he enters his second year of high school, she confesses to him that she wants to marry him. 

Shocked by this revelation, he confides in his female friend, Kaori Mishizuku, about his dilemma. 

From that moment on, he finds himself unable to avoid interacting with his stepsister, whom he had been avoiding ever since she expressed her desire to marry him. 

What will happen next?