Shirou's ordinary life as a poor construction worker gets turned upside down when he comes across a naked girl, Lucy, who's fallen out of the sky! Her strange and unique personality entices him and he introduces her to his apartment buddies. But after leaving for just a few minutes, he returns to the apartment to find Lucy gone, all his friends slaughtered and an ogre-like stranger standing amidst the carnage. The big man suddenly pulls Shirou out of the apartment building just before it explodes! Shirou gets pushed down into the sewage system of the city and is saved by a mysterious man...