In Natsumi Mizuki's absence, her parents left for London, leaving their 4th grade daughter in the care of her Aunt Izumi. Unfortunately for Natsumi, her solo night is interrupted by the sudden appearance of a baby named Mirai, whose mother is a 15-year older version of Natsumi herself. As an aspiring manga writer, Izumi has no time for the unexpected arrival but with the help of futuristic baby-care gadgets, they are able to raise Mirai secretly while maintaining Natsumi's normal 4th-grade life.