High school student Minato Usami is excited to gain a little brother when her father remarries. Likewise, Koshirō Sakuba looks forward to having a big sister after his mother remarries. However, when the two step-siblings finally meet, they discover each other to be quite different from the adorable images they had built up in their imaginations. This mismatch between expectation and reality forms the core of this home comedy, which follows the daily lives of the unexpected new siblings as they navigate their changing family dynamics.

The story takes place in a contemporary Japanese setting, focusing on the relationship between Minato, a high school girl, and Koshirō, her new stepbrother whose personality defies her initial hopes. Created by Tsubumi Mori, the manga is a slice-of-life comedy that explores the humorous and heartfelt moments that arise when two strangers are suddenly expected to live as brother and sister. Rather than focusing on dramatic conflict, the narrative arcs tend to revolve around the gradual process of the two getting to know one another, learning to communicate, and forming a new kind of familial bond despite their differences. The series has been recognized for its gentle and engaging storytelling, winning a New Face Award in the Manga Division at the 25th Japan Media Arts Festival and ranking on the Kono Manga ga Sugoi! guidebook list.