Elementary schooler Kei Hongo shares his home with his two-year-old sister Mai, a chubby-cheeked toddler whose adorable appearance masks a chaotic and unpredictable personality. Each day brings a new storm as Mai’s antics—her favorite word is a certain crude bodily function—send ordinary family life into a whirlwind of laughter, frustration, and outright disaster. Kei, an energetic first-grader, constantly finds himself on the receiving end of Mai’s mischief, but beneath the bickering there is a genuine sibling bond that surfaces in quieter moments. Their mother, perpetually exhausted but loving, sometimes turns into a “demon” when pushed too far, while their father is a gentle company employee who rarely earns Mai’s respect. Together the four Hongos navigate a never‑ending parade of everyday adventures: a card‑game tournament, a trip to the aquarium, Halloween festivities, a visit to Dad’s boss’s house, a pretend‑princess session, and the milestone of Mai’s third birthday. Along the way the series unfolds as an all‑ages family comedy that blends slapstick humor with heartfelt glimpses of family love, culminating in the revelation of Mai’s birth story and a finale that celebrates both the chaos and the warmth of growing up together.