Susumu Nakoshi is a middle-aged man who spends his days between a lavish first-class hotel and a park full of homeless people. One day, an eccentric young man approaches Nakoshi in search of subjects willing to partake in trepanation—the procedure of drilling a hole in the skull.

This mysterious person claims that Nakoshi is the perfect man for the experiment. Initially rejecting the offer, he doubles back on his decision after his car gets towed off. Nakoshi agrees to let Manabu Ito, a 22-year-old medical student, drill a hole in his skull in exchange for 700,000 yen. The procedure bears no results at first, with Nakoshi showing no abnormalities. This normalcy comes to an abrupt end, however, when Nakoshi begins seeing the homunculus in every person. With his perception distorted, how will Nakoshi face these homunculi?