Hiiragi, a high school student who works at a convenience store for a part time, was in trouble. When he is working alone at night, a monster-looking woman comes to the store. He can sense that she is not human...he was scared, so he decided to contact "THE GAP". He had heard that if you go on social media and post with the hashtag #THEGAP, someone will come and save you from monsters or ghosts. Hiiragi thought it might be one of those urban myths, but when he posted the hashtag, a beautiful woman visited him. Her name was Tsutsuji--the "Filler". She told him that there might be someone already dead in this case, but she'll help him out. Even Tsutsuji didn't know at that point...that Hiiragi would be the best partner for her job.