Atsushi Miyagawa is an 11-year-old boy with the appearance of a fully-grown man. His 17-year-old sister, Atsumi, has the stature of a child! From Atsushi being arrested numerous times as a suspected predator to Atsumi constantly grabbed like a small child more times than she can count, many things go wrong for this curious sibling duo. Atsumi's classmate is even in love with Atsushi, oblivious to his true age! Despite all this, indeed, there is not a single dull moment in the Miyagawa household.