Finally, the day of Mai's high school graduation has arrived. While Sakuta eagerly waits for his girlfriend, an elementary schooler who looks exactly like her appears before him. Suspicious, and for all the wrong reasons... Meanwhile, Sakuta and Kaede's father suddenly calls, saying that their mother wants to see her daughter. She was hospitalized because Kaede's condition had been too much for her to bear, so what could she possibly want now?

Puberty Syndrome—a rumored, mysterious syndrome that only affects those in their puberty. For example, a bunny girl suddenly appeared in front of Sakuta Azusagawa. The bunny girl's real identity is Mai Sakurajima, a teenage celebrity who is currently an inactive high school senior. For some reason, her charming figure does not reflect in the eyes of others. In the course of revealing the mystery behind this phenomenon, Sakuta begins to explore his feelings towards Mai. Set in a city where the skies and seas shine, Sakuta unfolds the meaning behind his bizarre encounters on women with the said syndrome.

In Fujisawa, Sakuta Azusagawa is in his second year of high school. Blissful days with his girlfriend and upperclassman, Mai Sakurajima, are interrupted by the appearance of his first crush, Shoko Makinohara.

After a draining December, Sakuta is quickly nearing the end of his second year of high school. Since Mai is a third-year student, they don't have much time left together before graduation rolls around. Meanwhile, his sister, Kaede, is slowly but steadily venturing outdoors again. Just as she begins to find her footing, she announces her most ambitious goal yet—attending her brother's high school! Sakuta knows better than anyone how difficult this will be for Kaede, and he's ready to support her however he can. He's just not sure if that's what's best for her...or what she really wants...