Mitsuba-kun wa Aniyome-san to.

Honjou Mitsuba (9 years old) had to move due to his asthma to a seaside town in Hamamatsu, Shizuoka Prefecture, where his brother and sister-in-law live.

However, since his brother is busy with his job and often not home, Mitsuba spends most of his time with his sister-in-law, Hinako (21 years old).

How will they manage to be a family while they barely know each other?