Masami is a high school student plagued by unexplained physical and psychological disturbances following the arrival of Rima, a transfer student from Mexico. Her initial collapse upon seeing Rima leads to a mysterious neck wound and recurring nightmares filled with religious symbols and violent imagery. Masami grows increasingly agitated by Rima's presence, unable to pinpoint the reason for her intense reaction.

Her condition deteriorates as the neck wound transforms into a fully formed mouth with sharp teeth, revealing a parasitic demonic entity. Unaware, Masami hosts this entity, which emerges nightly while she sleeps. It suppresses her consciousness, leaving her primary head blank and unaware as it feeds on Rima's blood. This supernatural manifestation is later interpreted as a physical embodiment of Masami's repressed jealousy toward Rima's beauty and popularity, effectively creating a Superpowered Evil Side.

To investigate her nocturnal episodes, Masami recruits a classmate, Umezu-kun, to record her sleep. Reviewing the footage horrifies Umezu-kun; he cannot describe what he saw and urges Masami to watch it. Viewing the video reveals the neck entity and its attacks on Rima, causing Masami profound distress and regret for seeking the truth.

As the entity continues feeding on Rima's blood, Masami undergoes a physical transformation, becoming noticeably more attractive—a side effect of the entity draining vitality from its victim. Attempts to repel the entity using ritual incense fail. The entity later fully consumes Rima in Masami's presence. This culminates in Umezu-kun's suicide after witnessing Masami's monstrous form approach him.