Koishikawa Mitsuki (voiced by Mariko Kokufuda) is a cute high school girl. She is lively by nature and thinks she has a happy and beautiful family. At the same time, the handsome boy Matsuura Yu (voiced by Ryutaro Okita) with a high IQ lives with his parents and lives an ordinary life of going to school and going home every day. One day, a joke of God brought the fate of the two children together. During a trip, Mitsuki and Yu's parents fell in love with each other, and the two open couples decided to form a special family of six. In this way, Mitsuki and Yu became nominal siblings. At first, Mitsuki was full of hostility towards this brother who often played pranks, but as she got to know him better, she found that Yu, who looked frivolous, actually had a very dedicated heart. Yu was gradually attracted by Mitsuki's frankness, and the two young hearts could not help but develop feelings. The various twists and turns that happened also made them understand that their love was so firm that even if they were accused of "incest", they would continue their deep love for each other to the end.