After waking up in her room on Christmas Eve, high school student Ryouka Tsurumi makes a series of chilling discoveries. Her mother informs her that her mansion has been evacuated and the area placed on lockdown due to the presence of an unknown and dangerous entity named X. The only other person left in the building is Shuuhei Murosaka, Ryouka's childhood friend. To add to the horror, Ryouka learns that X can brainwash people for its gruesome goal: feeding itself of human blood to reproduce.

However, Shuuhei comes to Ryouka's rescue and manages to save her from one of X's brutal attacks. As the two friends try to escape from the lockdown area, they are captured by hypnotized humans and subsequently devoured by the giant beast. But for Ryouka, the nightmare has just started: she wakes up for the second time in her room, only to find out that a different X is now threatening her life. Realizing that she is stuck in a time loop, Ryouka decides to team up with Shuuhei to find a safe future away from the threat of X.

[Written by MAL Rewrite]