Currently working at the resident service department of the district government, Fukazawa Hazuki (played by Nao Matsushita) leads an ordinary and peaceful life. As she is about to turn 30, she begins to consider marrying and starting a family with her boyfriend Yoshimura Tatsuya (played by Hiraoka Yuta), who also works at the district government. At this moment, an uninvited guest breaks into Hazuki's home, disrupting her original life. The visitor is none other than Hazuki's younger sister Misaki (played by Satomi Ishihara), who has been missing for many years after leaving home. Unlike the conservative and prudent Hazuki, Misaki, who has a cute face, is like a time bomb that can explode at any time. Her quirky personality may cause trouble. Sure enough, Misaki quickly ruined her sister's marriage. The appearance of the little devil makes Hazuki feel overwhelmed, but as the time they spend together slowly accumulates, the shining side of Misaki quietly emerges, and there is still an unknown secret in her heart. The little angel, or the little devil, caused a chemical change in the feelings of the two sisters...