A young girl named Rachel awakens at the bottom basement level of a closed-up building. She's lost her memories and has no idea why she's here.

Rachel is saved from an attack by Danny, the man calling himself her doctor, when Zack appears. She asks Zack to kill her, and he agrees as the two of them start to search for a way out of the building.

After arriving at the small room in the back of the reference room, Rachel encounters Eddie, the digger of graves and master of Floor B4. Eddie claims that he knows what happened to Rachel's parents and why she's in this building.

The door to Floor B3 opens to reveal a space like a prison cell, enclosed by pure white walls and iron bars. The moment Zack strikes the locked door before them with his scythe, the duo are barraged by machine gun fire.

Rachel finds the key card to get them out of the poison gas room, but Zack handles it too roughly and breaks it. The hourglass announces that their time is up, and a powerful poison gas begins to pour into the room.

After willingly shooting a dangerous drug into his arm so that they could move forward, Zack begins to grow overwhelmed by the urge to kill. But with the last traces of rationality left in his mind, he tells Rachel, "Don't let me kill you just yet..."

After slashing open his own belly, Zack is left without the strength to stand, so Rachel sets off alone into Floor B2 in search of something to stop his bleeding

Rachel heads to Danny's floor with Gray, the priest, to search for something she can use to treat Zack. However, Gray tells her she must undergo a series of tests to get there.