“But that’s what it’s like in the source material!” is a frequently used defense of subpar film adaptations, but here’s hoping Keiichi Sato and Yasushi Kawamura’s computer-generated “Gantz: O” represents a departure from the original “Gantz” comic (written by Hiroya Oku, adapted for screen by Tsutomu Kuroiwa). Otherwise it suggests strongly that all copies of the hugely popular manga should be shot into the sun. The film peddles an incoherent, paper-thin notion of heroism and a regressively gendered vision of an alien-monster-infested Japan, and while it’s possible that superfans might find it an acceptable addition to the canon and embrace its overtly video-game aesthetic, it seems unlikely to find much traction outside of the Japanese gaming/manga community. And those within it should really expect more, too.

The film opens mid-battle, as a young woman in a high-tech form-fitting catsuit cowers behind an overturned car in Tokyo’s Shibuya district. She is being menaced by a giant Orc-like monster — the irreconcilable clash between the fantasy aesthetic of the “Warcraft”-style creatures, the futurist sci-fi of heroes’ suits and weaponry, and the slick, contemporary locations is jarring from the off. The woman is Reika (voiced by Saori Hayami), who serves no narrative purpose except to have unfeasibly shiny, liquid hair and unfeasibly large breasts. Mystifyingly, the women’s combat suits appear to have the tensile strength to stop bullets and save the wearer from being crushed by giant troll-heads, but offer very little in the way of bosom support, resulting in a lot of localized jiggling.

Reika is saved by “teammate” Kurono (Yuki Kaji), who then determines to face the monster alone. “You don’t have to do it!” she wails. “Who else is there?” Kurono replies, and he’s right: There is no one around of the right sex and age bracket to make a viable hero. He kills it, but is also killed. Reika is sad, but her hair is great, and her breasts magnificent.

They are reluctant players in a game run by a large black orb that issues instructions and keeps score. Somehow, when you die (Reika was in a car accident, her ineffectual older teammate Suzuki, voiced by Shuichi Ikeda, had a stroke), you materialize, healed, in a featureless office where you meet your team, get your catsuit and get sent out against the clock to defeat a bunch of enemies. Fail and the whole team buys it. Succeed, and you accrue points which can be used to escape the game, upgrade your weaponry or resurrect a dead teammate (oops, there go the stakes!). But the dead Kurono was their leader, so it’s a good thing that in a Tokyo subway station, similarly strapping young male Kato (Daisuke Ono) gets viciously stabbed to death, and can be co-opted onto their team.

How the resurrections work, what happens to the dead bodies, how the players are chosen, what they do in between games, and who exactly they’re playing for remain deeply unfascinating mysteries. More fundamentally, the battles take place in real Japan and are reported on by the real news as unprecedented alien attacks. But the game has been going on a long time, so who or what did they fight before? And why have the players themselves, who are occasionally caught on camera, not been recognized and the game exposed? They can’t all have been so unlucky as to have had their sole close relative turn away from the television at the exact moment they were filmed katana-swording a big ugly beastie, can they? To think about “Gantz: O” at all is to overthink it.

The team are dispatched to Osaka, where they must face down not only myriad new monsters, including one who seems vanquished only for its eye to morph into a breast (which must be spectacular in 3D) as it takes the form of a giant naked woman made up of the writhing forms of hundreds of other naked headless women, they also face a rival Gantz team. But that does give Kato the chance to fall for Anzu (M.A.O), a romantically forward mother-of-one with a laser lasso weapon and a lovingly rendered thigh gap. While fighting monsters who are declared dead more times than cinema has been in 2016 (an argument for which ‘Gantz: O’ makes a compelling case), only to rise again with additional wings or horns or whatever, Kato still has time for the old boy-meets-girl, girl-dies, boy-wins-arcane-afterlife-game-and-uses-points-to-bring-her-back story.

The preposterous illogic, manufactured dilemmas, subterranean stakes, unearned sentimentality, and unquestioned sexism might be marginally more forgivable if “Gantz: O” looked good. But aside from some inventive creature design, the cheap, dead eyes and sterile plasticity of the CG humans suggests we’re no closer to bridging the uncanny valley than we were 15 years ago when the first all-CG “Final Fantasy” movie came out. No doubt the majority of the intended audience weren’t even born in 2001 and are accustomed to this visual style from their computer screens and console games. But this is supposed to be cinema, and if people are only as good as the art they consume, it’s a matter of some urgency to supply the teenage boys of today — the men of tomorrow — with something better than “Gantz: O,” a film in which its deemed OK, perhaps even witty, to have one of your two sole female characters referred to in the closing moments not by her name, but by her nickname: “Jugs.”