The Master Key (마스터 키, Maseuteo Ki) allows for special access to the Hidden Floor.
Several rules govern this "Floor":
- Revealing details about the Floor causes one to lose both the data they recorded as well as their memories.
- Only Regulars can enter. (Even Family Heads are no exception.)
- One cannot update their data or bring obtained items or goods from outside of the Hidden Floor once they have cleared the train.
Using the Master Key to enter the Hidden Floor allows Regulars to ignore the third rule, but prevents them from leaving any record.