Koyomi, the main character of the story, is a third year high school student who is close to failing every class except math.
Shortly before the story begins, he saved a vampire during spring break by giving it his blood, and became a vampire himself. Although Meme Oshino helped him become human again, there are a few lingering side effects; he can see in the dark and far distances, and he heals faster than the average human. He is still scared of morning sunlight even though it no longer has any effect on him.