In 2016, high school senior Mihane Yoshiyama is more focused on her hobby of photography than her impending university entrance exams. While investigating strange sounds in the school science lab, she is enveloped by the scent of lavender. From that moment, she discovers she has acquired the ability to leap freely through time. The bizarre occurrences in her days begin to center around a mysterious new classmate and childhood friend she does not remember, a boy named Fukamachi. This new student is not who he appears to be; he is actually a researcher named Ken Sogoru from the year 2122 who has traveled to the past to study summer. After losing the time-travel drug he developed, he has modified the memories of those around him to pose as the teenager Fukamachi while he works to replicate his formula.

As Mihane learns to control her newfound power, initially using it to satisfy small whims and avoid awkward moments, she becomes entangled in a much larger web of consequence. Using her ability to jump through time, she investigates the origins of a classmate who received a heart transplant, discovering the donor was a brilliant young woman whose unspoken love for a teacher ended in tragedy. Mihane attempts to alter the past to save the donor, but learns that changing one event will prevent the transplant that saved her classmate's life, forcing her to make a painful choice to preserve the present.

The central conflict deepens as Mihane develops genuine feelings for Fukamachi, the boy from the future. Her true childhood friend, Asakura Goro, has long harbored feelings for her and planned to confess during the summer festival, but finds himself perpetually outmaneuvered by the enigmatic transfer student. As Fukamachi and his colleagues from the future prepare to return to their own time, the drug enabling their journey begins to lose its potency. The future researchers reveal to Mihane that all her memories of Fukamachi were artificially created modifications, and the childhood bond she treasures never existed. Faced with the erasure of their relationship, Fukamachi confesses that despite the artificial origins of their connection, his love for her became real. He returns to the future, taking with him all evidence of his existence from Mihane's memory.

The narrative arc concludes in bittersweet ambiguity. Mihane is left with only fragmented impressions of the summer and her camera full of photographs she cannot fully explain. She retains the emotional impact of her experiences and the maturity gained from confronting love, loss, and the heavy responsibility of altering time, even as the details of the boy from the future fade from her mind like a dream upon waking.