Mai and Hikaru went to school together in the city of Nagoya, until Hikaru disappeared when they were eighteen.
It is not until ten years later, when Mai runs into Hikaru's mother, Hiromi Sato, that she learns Hikaru has become a hikikomori, a recluse unable to leave his bedroom for years. In secret, Hiromi Sato hires Mai as a 'rental sister', to write letters to Hikaru and encourage him to leave his room.
Mai has recently married J, a devoted salaryman with conservative ideas about the kind of wife Mai will be. The renewed contact with her old school friend Hikaru stirs Mai's feelings of invisibility within her marriage. She is frustrated with her life and knows she will never fulfill J's obsession with the perfect wife and mother.
What else is there for Mai to do but to disappear herself?