An aspiring novelist and university student named Ai Tanaka decides to find a roommate through a classifieds website to share living expenses. She connects with Miu Yamaguchi, a seemingly plain woman with long black hair who claims to attend the same university. What begins as a practical arrangement soon descends into psychological horror as Miu's behavior becomes increasingly disturbing and parasitic.

The story is set in contemporary Japan, focusing on the claustrophobic confines of a shared apartment where Ai finds herself trapped with a dangerous imposter. Miu's odd habits quickly escalate from borrowing things without permission to copying Ai's physical traits, such as drawing moles on her face to match Ai's own. Suspicion turns to terror when Ai sneaks into Miu's room and makes a horrifying discovery, forcing Ai to realize that her new roommate is methodically attempting to steal her identity and her life.

The larger narrative is framed by a gruesome opening scene involving a corpse whose face and fingerprints have been deliberately destroyed, signaling to police that a deeply disturbed mind is at work. This early act of violence hints at the broader murder spree connected to Miu's past. As Ai attempts to escape the situation by contacting Miu's mother for answers, Miu visits her own parent with a dangerous weapon, raising the stakes considerably. The story transitions from a tense psychological mystery into outright horror following a specific act of violence around the sixteenth chapter, which shifts the focus toward traumatic consequences and survival. Key narrative threads include Miu's childhood trauma, the fallout from Ai's relationships, and the question of whether Ai can use her novel-writing ambition to make sense of the nightmare unfolding around her.