Just before summer break, Sakuta Azusagawa encounters a middle school girl named Shoko Makinohara, who has the same name and face as his first love. The problem is that his first love should already be in college, making this meeting seemingly impossible. Before he can unravel that mystery, another strange phenomenon strikes. His friend Rio Futaba, the sole member of the science club known for her logical and impassive demeanor, discovers she has been split into two separate, physical doppelgängers.

One Rio continues living her normal life, while the other is forced to leave her home and ends up staying at Sakuta’s apartment. Both versions share the same memories and personality up to the point of the split, but being physically separated forces them to follow different paths. Neither can explain what has caused this sudden case of Adolescence Syndrome, a supernatural phenomenon where intense emotional turmoil and social pressure manifest as real-world disturbances like time loops, invisibility, or in this case, a duplicate person.

Complicating matters is Sakuta’s girlfriend, Mai Sakurajima, a celebrity who decides she will also sleep over at his place, turning his summer vacation into chaos. As Sakuta works to help Rio, he discovers that the two versions of her might be linked to her secret insecurities. The apparently logical Futaba has been posting revealing photos of herself online, a behavior driven by a desire for external validation that conflicts with her rational self-image.

The central conflict revolves around the Rios belief, influenced by quantum mechanics, that the only way to end the phenomenon is for one version of herself to disappear or die. One of the doppelgängers moves in with Sakuta, leading to awkward living arrangements and forcing difficult conversations about identity and self-worth. The story explores how Rio struggles with her feelings for their mutual friend Yuuma Kunimi, who is already in a relationship. Meanwhile, the unresolved subplot about the younger Shoko Makinohara, who has a heart condition, continues to loom in the background, adding further emotional weight to Sakuta’s summer. The climax resolves when the two Rios confront their fears of rejection and self-loathing, allowing them to reintegrate into a single person without the tragic outcome she had feared.