As any neurologist could tell you, that old tale about how humans only use 10% of our brains is a load of bunk, but the idea continues to exert a fascination within the realms of pulp entertainment.

In Hideo Yamamoto’s manga series “Homunculus,” this untapped cerebral potential is unleashed through a method based on even shakier science: the skull-perforating technique known as trepanning. But rather than transforming people into genius superheroes, like Scarlett Johansson in 2014’s “Lucy,” the procedure invests them with psychic powers — as if the hole drilled in their foreheads was to make way for a third eye.

This silly concept is given a decidedly low-IQ treatment by Takashi Shimizu’s screen adaptation, which opens in Japanese cinemas before a worldwide release on Netflix.

Run Time 115 mins.
Language Japanese
Opens Now showing

Go Ayano is at his most grizzled playing Susumu Nakoshi, who we first find living out of a car parked on the border between Tokyo’s highrises and a park populated by homeless people. He’s not sure how he got there, but his disheveled suit and still-valid American Express card suggest he fell from a great height.

All the same, Susumu submits a little too easily to become a guinea pig for Manabu Ito (Ryo Narita), a wild-eyed medical student with the fashion sense of a feral K-pop star.

After going under the drill, Susumu finds he is able to see other people’s “homunculi”: visual manifestations of their buried traumas, which look more like the supernatural creatures of Japanese folklore. The first time he discovers this ability, it’s like something out of Hayao Miyazaki’s animated 2001 hit “Spirited Away,” but the film never recovers that initial sense of wonder.

Squeezing Yamamoto’s 15-volume series into a feature film was always going to be a challenge. Shimizu’s script — co-written with Eisuke Naito and Naruki Matsuhisa — sticks fairly close to the first half of the story, in which Susumu uses his powers to perform a sort of brute-force psychoanalysis on unsuspecting strangers.

Regrettably, this includes a sequence in which he “cures” a high school student of her pathologies by sexually assaulting her, which somehow manages to be even more unpleasant here than it is in the manga.

So it’s odd that the film shies away from the loopy excesses of the manga’s latter half, in which gender and physical appearance prove to be just as mutable as the homunculi themselves. Instead, it cobbles together a bit of sub-Hitchcock intrigue involving Yukino Kishii as a woman from Susumu’s past, while reducing Manabu — who’s far more complex in the original, not to mention gender fluid — to a simple villain.

The longer the film goes on, the harder it is to say who “Homunculus” is for. Viewers who aren’t turned off by that rape scene (and they really should be) will probably feel cheated by the sudden pivot to conventionality, culminating in an ending that’s so benign, it must have been producer-mandated.

Yamamoto is best known as the author of “Ichi the Killer,” which spawned a boundary-pushing 2001 movie by Takashi Miike. I doubt Shimizu has it in him to make a film like that, but “Homunculus” feels like a poor compromise. If you’re going to start drilling holes in people’s heads, you need to be prepared to go all the way.

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