13-year-old Rachel "Ray" Gardner is taken to a hospital for counselling after witnessing a murder. However, she wakes up to find herself on basement Floor B7 instead with no memories apart from her name and the reason she came to the hospital. A series of mysterious broadcasts and scribbled messages on the wall set the scene as a game where each participant is designated a floor of their own, and anyone who trespasses on another participant's floor has the chance to be killed.