In order to maintain the tengu abilities in his bloodline, the protagonist has to marry his own little sister. However, a problem has arisen, because it turned out that there was a possibility his little sister and the branch family's daughter were switched at the hospital. As such, the branch family's daughter has been adopted into the main family, becoming the protagonist's second little sister.

Michiru, the little sister who grew up with the protagonist used to act coldly towards the protagonist, but the moment a rival appears and is ready to do anything to her new brother, she decides to fight back.