Susumu Nakoshi is a middle-aged man who divides his time between a luxurious first-class hotel and a park populated by homeless people. One day, an eccentric young man approaches Nakoshi looking for subjects willing to undergo trepanation—the procedure of drilling a hole in the skull. This mysterious individual claims that Nakoshi is the perfect candidate for the experiment. Although he initially rejects the offer, he reverses his decision after his car is towed away. 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 yields no results at first, with Nakoshi showing no abnormalities. However, this normalcy comes to an abrupt end when Nakoshi begins seeing homunculi in every person. With his perception distorted, how will Nakoshi face these homunculi?