When his parents embark on a business trip, Shuuto Sakaguchi anticipates a carefree three months ahead. Yet, his return from school unveils a surprising encounter with Chiho Norita, the daughter of his parents' associate. Tasked with looking after Chiho against his wishes, Shuuto grapples with his inexperience in childcare. Over a hundred days, an unlikely bond blossoms between Shuuto and Chiho, intertwining their lives in unexpected ways.