Luka Kobayakawa is one half of identical twin sisters, both members of their school's track team. Each twin harbors romantic feelings for the same upperclassman, Katsuyuki Touma. His choice of Luka over her sister Rumi sows underlying tension between them.

A pivotal event unfolds when the track team takes shelter in a cave, later revealed as a mass grave. A strange smell within kills the entire team except Luka and Rumi. Both sisters become infected with an unusual bacterium during this incident, triggering significant physiological and psychological changes.

Following the infection, Luka develops terrifying psychic abilities. Concurrently, Rumi undergoes a dramatic personality shift, becoming intensely possessive of Katsuyuki and manifesting homicidal tendencies specifically aimed at Luka. Despite witnessing Rumi kill a classmate and attempt her own murder, Luka exhibits persistent reluctance to accept the full extent of her twin's transformation into a homicidal threat. This denial stems from their shared history of trust and closeness before the infection.

Forced into survival mode as targets of Rumi's aggression, Luka and Katsuyuki flee together. This shared danger deepens their bond and mutual reliance. Throughout her ordeal, Luka is characterized by trust, sometimes appearing gullible or naive, particularly in her struggle to reconcile Rumi's past supportive nature with her present murderous persona. Luka's journey involves navigating the trauma of the cave incident, coping with her new psychic powers, confronting the horrifying reality of her sister's actions, and protecting her fledgling relationship with Katsuyuki from Rumi's obsessive interference.