Kotori Izumi lives as an ordinary high school student in modern Japan, sharing a close bond with her childhood friend Shin Hazama. Her father heads Izumi Heavy Industries, where Shin's father worked before his death. Kotori consistently shows concern for Shin following his loss, offering emotional support despite his tendency to withdraw. Subtle romantic feelings emerge through their interactions, including an accepted date invitation occurring alongside intensifying interdimensional conflicts.

Kotoko, Kotori's counterpart, exists in a parallel Japan where feudal hierarchy merged with advanced technology. As princess, Kotoko rules under an oppressive regime known for public executions of perceived threats, including Jin Hazama's parents. This rulership initially presents as autonomous and tyrannical, casting her as the primary antagonist.

The characters' fates are intrinsically linked: any death in one reality instantly causes the corresponding death in the other. This connection drives Jin's mission to assassinate Kotori, believing her death will eliminate Kotoko and free his world. Princess Kotoko deploys robotic enforcers, Miko and Riko, to protect Kotori and preserve the existing connection.

Later revelations expose Kotoko's rulership as partially controlled by external forces, including the scientist Yūri operating behind the scenes. Evidence indicates Kotoko's public brutality concealed covert resistance against shadowy organizations manipulating her society, reframing her actions as strategic rather than purely despotic.

Their trajectories converge when Kotoko makes a voluntary sacrifice to sever the worlds' connection. This decisive act ends both her life and Kotori's latent existential link while preserving Kotori's existence in the primary reality. The resolution allows Kotori to continue her relationship with Shin free from interdimensional threats.