Homunculus tells the story of a middle aged man, who spends his days
living in a car, smack inbetween a park infested with homeless people
and a large, upper-class hotel complex. He has nothing, yet he is
always classily dressed and looks well taken care of, despite the fact
he sleeps in his car. Once a day, he always takes the car out for a
ride, no matter what the weather. One day, an oddly dressed young man
approaches him, and asks him if he would like to participate in a
little experiment... Involving having a hole drilled in his skull. He
promptly refuses; he has no need for money anyway. The next day,
however, his car runs out of gas and gets towed. Desperate for money
to get his car back, he approaches the young man and accepts to have
the hole drilled in his skull. ''A safe procedure.'', he is assured.
But then, of course, that's when the side effects kick in...
synopsis from Anime News Network