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After cleaning the school chemistry lab, Kazuko Yoshiyama smells lavender from a mysterious liquid and faints. A few days later she and her classmate Goro are about to be struck by a truck, and she is thrown back to the previous morning. She has to relive a day she has already seen, including the earthquake and the fire at Goro's house.
Her classmate Kazuo Fukamachi turns out to be a visitor from the distant future. He came to collect lavender to make a time-travel drug, because plants have become scarce in his era. He fit into the present by giving everyone false memories that he had always been their friend, so a stranger passes as someone familiar.
Before Kazuo returns to his own time, he erases his existence from everyone's memory, including Kazuko's. She still keeps a faint pull toward the scent of lavender and a feeling that someone important will come back. The novel suggests that feeling persists after the facts are deleted.
Kazuko does not simply use her power to dodge accidents. She confides in her science teacher, Mr. Fukushima, and together they reason that she must go back to the moment the anomaly began. Returning to the lab on that day is what reveals Kazuo's secret. The lens is that a strange effect is understood by rewinding to its causal origin rather than by managing its symptoms.