Serenity certainly deserves an A for effort. At a time when studios and streaming services are doubling down on sequels and reboots and endless rivers of interchangeable chum, it’s rare to see a movie like this one appear in theaters: a mid-budget thriller with a pedigreed cast that isn’t based on any existing I.P., the sort of project the A.A.R.P. might call a “movie for grown-ups.” Writer-director Steven Knight should sleep soundly knowing that his latest isn’t just another gear in the ever-churning content turbine—it’s a wholly original creation, one that Knight has clearly put his heart and soul into.

Then again, that description hardly does justice to Serenity, a film so magnificently ill-conceived that it instantly earns a spot alongside I Know Who Killed Me and The Number 23 in the pantheon of great terrible movies. And that’s even before the emergence of a late-stage left-hand turn that throws the entire movie into twisted relief.

It’s rough enough at the start, when Serenity leads you to believe that it will follow the misadventures of a burnout fisherman named Baker Dill (!), who’s on a vain quest to catch an evasive giant tuna he calls “Justice” (oh boy). The burnout is played by a slurry, blurry Matthew McConaughey, chewing scenery as if the McConaissance were nothing more than a collective delusion. This self-serious Moby-Dick riff becomes an over-the-top neo-noir soap opera when the fisherman’s ex-wife, Karen (his onetime Interstellar costar Anne Hathaway, sporting Jessica Rabbit hair and a sultry pout), shows up and offers him $10 million to murder her abusive new husband, Frank (Jason Clarke, somehow going even bigger than his co-stars, who are already doing the most).

There’s ham-fisted dialogue and gratuitous violence and furtive boat sex, and so many shots of McConaughey’s bare butt that each cheek deserves its own supporting credit. At one point, Karen tells Dill that their young son can somehow hear him “through the computer” when Dill absently speaks aloud to the kid, and Dill hardly seems fazed. It’s clear, especially from this point forward, that something is up, that Serenity is more than the over-boiled melodrama it appears to be—and not just because the movie does its best to convince us that the characters played by 49-year-old McConaughey and 36-year-old Hathaway are supposed to be roughly the same age.

And then—well, to reveal what we learn next would rob the movie’s hall-of-fame-insane twist of its audacious power, so stop reading here if you have any intention of actually seeing Serenity. If you want to get the payoff without doing the work? Stick around, and buckle up.

So: does it turn out that Dill has been dead the whole time? Does he have a telepathic connection to his estranged child, or a case of multiple-personality disorder? Is the mysterious island where he lives actually purgatory, or a nightmare, or a hallucination, or a figment of an autistic, snow-globe-loving boy’s wild imagination? Is somebody, anybody, secretly a robot?

No! But sort of. Because the truth is, Baker Dill isn’t real. Neither is Karen. Neither is anybody on Plymouth Island. See, Dill et al. are actually characters in a video game that was designed and programmed by a disturbed 13-year-old Miami native named Patrick (Rafael Sayegh), the son of a deceased Iraq War hero (who looks an awful lot like Matthew McConaughey) and a battered woman (who looks an awful lot like Anne Hathaway) who is indeed abused by her new husband (Jason Clarke, natch). Patrick originally created the game so that a version of his father would live on in a gentle, nonviolent environment—but has decided to change its code so that the avatar of his father can murder the avatar of his stepfather, because Patrick has decided he wants to kill his actual stepfather in real life.