グロ・でれ

One day, on his way home, Komura Kazuto ran into a girl from his past; a girl he had not seen in 10 years. 

But this was no normal reunion. The girl was standing in the middle of the road, about to be hit by a truck. Kazuto rushed to her side and pushed her out of the way, then... everything turned pitch black.
When Kazuto regained consciousness, the girl was by his side. 

The girl's name was "Ilmi." She had apparently saved Kazuto at the last minute. 

Ilmi asked Kazuto if he could let her crash at his place for a bit, and he could not find it in himself to turn her down, bringing her home with him. 
That night, Ilmi snuck into Kazuto's bed, and the two ended up sleeping together. For some reason, Kazuto lost consciousness right after. 

And the first thing Kazuto saw when he woke up again was Ilmi sitting naked in front of him, holding her own severed head in her hands. With a bright smile, Ilmi then said:
"You really know how to punish a girl."
Shocked and confused, Kazuto asked her how she could possibly still be alive, and with a brilliant smile, she answered:
"Because I'm a vampire, duh!"

This was the beginning of Kazuto and Ilmi's lovey-dovey, lewd and grotesque relationship.

- Translated from the official summary by Dergonu