Sai had always grown up to be a good child, not doing anything bad and she loved her family a lot. Circumstances changed for her when she reached graduation, and she decided to move away from home. Coming to live in the city in her own apartment, Sai soon finds an enjoyable, quiet life. She doesn't know from where it came, but a danger presents itself in her city. A toxin spreads, killing many, and Sai is forced to live trapped inside her home, not being able to go out very often. And she also must live with tough decisions along the way.