From Good Girls Scan BL:

Writer Hasegawa Mitsuru has been investigating a case that was rumored to be the work of vampires known as “Biters”. His childhood friend, Annaka Yuusei, a.k.a. Anna, warns him not to stick his head into where it doesn’t belong. (SPOILER) But the truth is, Anna is actually a Biter. Under the pretense of gathering information for his article, Mitsuru agrees to let Anna suck his blood. On top of that, he impulsively sleeps with Anna and experiences indescribable pleasure. In addition, there seems to be a secret about Mitsuru that he himself isn’t aware of…?