Umino Nagi, mistakenly raised by another family, finally meets his biological parents in high school. On this fateful day, he crosses paths with Amano Erika, a student from a prestigious girls' school. To avoid an arranged marriage, Nagi reluctantly pretends to be Erika's boyfriend. However, a startling revelation unfolds—they were the switched babies and are now betrothed! Embracing this twist, their families endorse the engagement, leading the duo to cohabit. While Nagi harbors feelings for classmate Segawa Hiro and Erika rebels against her father, they agree to keep their relationship platonic. Thus commences a whirlwind of mixed-up and uproarious romantic entanglements!