Fifteen-year-old Kazuko Yoshiyama is cleaning the science lab at her middle school when she inhales a strange lavender-like scent and faints. After this incident, she begins to notice odd occurrences around her, including the prediction of an earthquake that leads to a neighbor's house fire. When she narrowly avoids being hit by a truck only to find herself waking up in her bed the previous morning, Kazuko realizes she has accidentally gained the power to leap backward through time.

With the help of her two close friends, Kazuo Fukamachi and Goro Asakura, Kazuko tests the limits of her new ability. Their science teacher explains her power as a combination of teleportation and time-leaping, though the true source remains a mystery. As Kazuko experiments with her leaps, she learns that each jump consumes a limited number of chances, forcing her to consider the consequences of altering past events.

The story takes place in a seemingly ordinary Japanese middle school setting during the 1960s, though the mundane backdrop contrasts with the extraordinary discovery Kazuko has made. Her world expands beyond typical adolescent concerns when she makes a four-day leap backward to the science lab where her powers originated. There, she encounters a mysterious young man who has been pretending to be her friend Kazuo.

This stranger reveals himself as Ken Sogoru, a time traveler from the year AD 2660. Ken explains that Kazuko acquired her time-leaping abilities accidentally through exposure to a drug he brought from the future. Rather than being a special gift, her power is a side effect of his temporal journey, and her repeated leaps have been consuming the limited energy of his device.

Ken decides to remain in Kazuko's time for one month, and during this period, she falls in love with him. However, when the time comes for him to return to his own era, Ken erases all memories of himself from everyone he has met, including Kazuko. As the novel closes, Kazuko retains only a faint, lingering impression of someone who promised to meet her again, triggered whenever she smells lavender. The story combines science fiction elements with the bittersweet emotions of first love and the realization that some meetings, no matter how meaningful, are temporary.