Based on the 2019 television drama adaptation of the 1980s manga, Hiromi, a timid and quiet high-schooler, was forcefully raised by her grandmother. Due to her abuse, Hiromi created an imaginary version of herself in the mirror, named "Yumi," and would play by talking to her as a child. By the time Hiromi becomes a high school student, she only remembers playing with the mirror, but one day, she hears a voice coming from inside her reflection asking, "Is what Grandma says really right?" Triggered by this voice, Hiromi's alternate personality freely controls Hiromi's body and ventures out into the night without Hiromi's knowledge...what sort of dangerous activities will "Yumi" force her timid host into, and what will become of this dynamic?