Two high school girls, Airi Sezaki and Hinako Hanamura, who were friends during middle school, now belong to different social groups and pretend not to know each other in class. However, after school, they secretly meet in the science lab where Airi initiates a series of escalating fetishistic acts with Hinako, who reluctantly plays along out of a desperate fear that refusing would mean losing Airi's friendship and affection. Their clandestine relationship is discovered by a classmate named Yoshimi Matsushita, who takes shocking photographs and uses them to blackmail the two girls.

The exposure leads to social ruin for Airi, who denies any involvement with Hinako and stops their after-school meetings. One of Airi's former friends confronts her about bullying Hinako, and the argument devolves into screaming. Airi stops attending school altogether and becomes disheveled and withdrawn in her home. Hinako, worried, tracks her down and begins visiting daily. The power dynamic between them flips, with Hinako now taking control in a relationship that has become thoroughly codependent and toxic.

Another classmate, Ichika Miyazono, reaches out to Airi, motivated by a desire to become an important side character in what she sees as Airis story. She secretly plants a spy camera in Airis room, but Hinako, who has done the same thing, discovers it and chases her off. Hinako then persuades Airi that no one but her can be trusted. Believing this, Airi runs away from home, and Hinako joins her. Using money saved from birthdays and holidays, they travel to a nearby town and hop between hotels.

When their funds begin to run out and a police search begins, Hinako finds a job at a local bookstore. Airi secretly resents this development, becoming consumed by the fear that Hinako will abandon her and that their relationship is one-sided. Feeling helpless and dependent, Airi contemplates suicide. She takes Hinako to a beach and persuades her to wade into the freezing water with the intention of dying of hypothermia, but breaks down, screaming that she does not want to die. The two return to their hotel, where Airi confesses she would die without Hinako, who promises to stay.

Later, Airi discovers that Hinako secretly texted her mother asking for money. The two argue before settling. That night, Hinako asks Airi how she thinks something becomes eternal, then answers her own question: something becomes eternal when it ends. When Airi asks if she will make them eternal, Hinako places her hands around Airis neck and begins to strangle her. She reflects on her hatred of how Airi only interacted with her when convenient, how she wanted Airi all to herself, and how she wanted the old, lively Airi back. Unable to stop herself, Hinako continues until Airi dies. She carries Airis corpse on her back out into the snow, trips, and succumbs to hypothermia as she reaches to hold Airis hand. The final image shows the two girls lying together under a blanket of snow.