Mitsuno Hiiragi is an ordinary high school student who works part-time at a conveni, but he's troubled by a strange-looking woman with a contorted body who visits the store every night. Desperate to do something about it, Hiiragi puts the rumor that's been going around town to the test, half-believing it, and is greeted by a mysterious beauty named Tsutsuji who calls herself the “Filler”. “Sukimamono” (gap monsters) are strange creatures that have strayed into the human world, and it seems that Tsutsuji makes a living by “doing something” to them… She's extremely beautiful, but no matter how you look at her, she's suspicious, shady, and just strange…! Swept away, Hiiragi tries to find out the woman's true identity together with Tsutsuji, but…