The truly terrifying mutation does not come suddenly, but erodes daily life bit by bit, so subtle that even the person and the people around him are not aware of it. By the time they notice it, it is too late, and the situation has progressed to a hopeless point... Four ordinary female students were laughing and playing around one second, but took up weapons to fight with unknown monsters the next. Surrounded by the anxiety of not being able to contact their families and the fear of the unknown world, the darkness of human nature will be infinitely magnified. Where should they go? Whether they live or die, it all depends on a split-second decision...