When Ryoka, a high school girl, wakes up, she is left behind in a sealed-off apartment. A message from her mother tells her that there is a mysterious creature called "X" in the apartment.

When Ryoka hears that her childhood friend Shuhei is also left behind in the apartment, she calls him. What is the ecology of "X" that her mother told Ryoka as she waits for Shuhei to join them?

X has good hearing and walks on two legs. Ryoka is perplexed by the fact that X has different characteristics than what she saw in her dream, but first she heads to the fourth floor where Shuhei is waiting to meet up with her.

Thanks to Shuhei's quick thinking, he manages to distract X's consciousness. However, X's pursuit does not stop, and Shuhei is eventually captured by X.

Ryoka and Shuhei make use of X's blindness and reach the elevator safely. What awaits the two on the 31st floor, which should not exist?

Ryoka and Shuhei have made it to the 31st floor, which shouldn't exist in the apartment building, and have cleared level 2.

Ryoka discovers a candy house. There, she finds a witch who is a level 3 X. Ryoka is thrown into a loop again and heads off to confront the witch.

Ryoka and Shuhei fight the witch to save the captured Hansel and Gretel. The witch uses spider webs to immobilize the four of them.

After defeating the witch, Ryoka and Shuhei take the elevator to the 32nd floor.

Without any clues from the notebook, Ryoka and Shuhei challenge Level 4. Ryoka is driven to despair by the fear of being killed again.

Having grasped the true identity of X, Ryoka and Shuhei make a desperate attempt to conquer Level 4.