Lockdown Zone: Level X Season 1

Ryoka wakes up to a long list of missed calls from her mother, who then explains to her the current situation.

Ryoka hears that her childhood friend Shuuhei has also been left behind in the apartment, so she calls him. As she waits for Shuuhei to join them, Ryoka's mother tells her about the biology of "X (Sai)"...

Ryoka's mother explains to her about the threat in the building, similar to her dream, this time Sai is a walking monster that responds to noise.