Mamiko Abe is a high school student roughly two years older than her boyfriend, the genius inventor Yutaka Takai. She grows alarmed when Yutaka builds a time machine aiming to travel two years into the past to become her age. Seizing an opportunity while Yutaka is distracted, Mamiko takes the machine herself. She sets it for two years into the future and plunges from the school rooftop, calling "Wait for me" to Yutaka as she vanishes.

Her disappearance initiates a slow fading of Mamiko from everyone's memories, including her family. Yutaka initially retains his memory of her and fights to preserve it, but ultimately forgets her entirely. The story culminates when Yutaka instinctively remembers Mamiko moments before her scheduled return two years later. He races to the school rooftop and catches her, preventing her fatal fall upon arrival and securing her survival and reintegration into the timeline.

Mamiko's role focuses on her relationship with Yutaka and her crucial act of using the time machine, driving the narrative's examination of memory and consequence.