Unami Mamori, an ordinary second-year high school student, has a stepsister, Unami Shikino, from their parents' remarriage. Their sibling relationship began when Mamori was in fourth grade and Shikino was in third, but they weren't close at all; rather, it was quite the opposite.

Whenever Mamori tried to bridge the gap by inviting her to play or do something together, Shikino would always silently distance herself. Even when he tried to start a conversation, he was always met with a curt attitude, and Mamori had resigned himself to the fact that building a good relationship with his stepsister was impossible. However, one day, an incident occurs. Shikino had gotten her first-ever boyfriend since entering high school, but he was stolen by another girl.

Desperate, Shikino withdraws into her room. Seeing his stepsister like that, Mamori couldn't remain silent and began to comfort Shikino.

"The only one I can trust... is you, big brother..."

"Huh...? Sh-Shikino...?"

"I love you... I have, ever since I was little... always..."

"Wh-What!?"

Thus begins a sweet-and-spicy, triumphant rom-com, featuring an ordinary protagonist who gets things done when it matters, and his stepsister who's a little shy about confessing her feelings!