At only 11 years old, Atsushi Miyagawa possesses the physical features of an adult while his older sister, Atsumi, has a childlike appearance despite being 17. These peculiarities lead to a series of mishaps, including Atsushi being wrongfully arrested as a potential predator and Atsumi being treated as a small child on numerous occasions. To add to the chaos, Atsushi is adored by one of Atsumi's classmates who remains unaware of his true age. Despite the challenges, the Miyagawa household is always buzzing with excitement.