Natsumi Mizuki, a fourth-grade student temporarily living alone during her parents' travels, faces a dramatic upheaval when her future daughter, Mirai, unexpectedly arrives from the year 2007. Mirai's time travel results from a lightning strike affecting a communication device. Suddenly responsible for an infant while juggling school and friendships, Natsumi struggles.

Initial reluctance and conflict arise with her aunt, Izumi Shimamura, an aspiring manga artist who moves in as a temporary guardian but resists childcare duties. Motivated by a plea from her future self to protect Mirai, Natsumi's determination gradually transforms their dynamic into a reluctant partnership. They work together to conceal Mirai from friends, neighbors, and authorities to prevent time paradoxes, including Natsumi devising plans to retrieve Mirai after a clinic mix-up.

Natsumi confronts practical challenges: learning infant care, managing household tasks, and balancing school obligations. She discovers Mirai's crying oddly aids her flute practice, leading her to bring Mirai to school for a music performance. Her growth surfaces in actions like venturing out at night to find Izumi during an argument or seeking childcare advice from her grandmother. Crises, such as briefly losing Mirai in public, deepen her commitment despite exhaustion.

Key moments illustrate her evolving maturity: wearing her mother's wig to pass as an adult during outings, preparing meals to impress a crush's family, and speculating about Mirai's father based on the infant's attachment to classmate Daisuke. These experiences collectively shape Natsumi from a reluctant caregiver into a devoted maternal figure, highlighting her adaptability and resilience.