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?

Ryoka and Shuhei manage to escape from "X" just in the nick of time. They try to leave the apartment, but suddenly lose consciousness.

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 reached the 31st floor, which should not exist in the apartment, and cleared level 2. Ryoka and Shuhei were about to go to Level 3 when a strange girl came into Shuhei's room...?

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