Taraji P. Henson as a gang assassin with a conscience who ends up fighting both sides of a turf war? Fucking-a yes. A neo-blaxploitation film with a classic soul soundtrack and posters like these? Not as much of an enthusiastic yes, but sure, why not. Both of those films in the same package? Obviously yes, nothing could be more natural.

So how is it, then, that Proud Mary is so irredeemably awful? Because it is all of those things, and on top of it, it has a real swell opening credits sequence, in which a curlicued '70s-looking font (the same one on the posters) parades across on the backs of cherry-red "swoops", while Henson gets dressed to be a badass, and her face replicates across the screen in multi-colored lines. It's pure disco-era cheese, and it's vivid and beautiful and it promises, at the very least, a film that doesn't utterly violate all the most basic rules of cinematic grammar as they have been developed over the last 110-odd years of world filmmaking. In this regard, Proud Mary breaks its promise.

Seriously, I'm not sure how to explain it. This is a movie that cost $14 million dollars of real studio money, from Sony Pictures via that company's Screen Gems label. I would have assumed that those facts alone would demand a certain minimal level of competence; I would have assumed, if I could not persuade myself to assume anything else, that $14 million at least means you can afford some ADR sessions. But then along comes a scene shot outside in a park, where all of Henson and co-star Jahi Di'Allo Winston's lines are fuzzed out with wind noise, and then a god-damned airplane comes along and obscures their lines even more, and I guess it turns out that my assumptions were all wrong, because here comes a wide-release multi-million-dollar studio film with sound errors that would earn any film student in the country a C grade and a stern note in red pen.

And that's merely the point where it's so bad that it's basically comic. The film fails on all counts. Eyelines frequently don't come any closer than the most loose approximation of meeting up. Cutting ignores any sense of spatial continuity; there are moments when what is self-evidently the wrong establishing shot, chronologically, is used; the film has a montage of fades to black, the kind of thing so wrong-headed that "montage of fades to black" is a contradiction in terms. Cinematographer Dan Laustsen's interiors are almost always under-exposed, except for a couple of times where windows blow out in places that I cannot fathom they were meant to (and Laustsen shot John Wick: Chapter 2; if there is one specific task we know he can do well, it's light the stylised interiors of an action film).

In one scene, there's grey at the bottom of the frame. You can figure out what it's supposed to be: it's supposed to be fog. But as an element of composition, it's just a perfectly straight strip of blurry grey, covering the bottom 10% of the frame. Why is it there? Why did nobody on set propose to film maybe one version of the shot that didn't look like a technical error? Why didn't anybody in post-production silently zoom in the shot clear that bit out? I cannot say.

The script buried beneath what I'm already prepared to call the worst filmmaking of 2018 - oh God, please let it be the worst filmmaking of 2018 - isn't as objectionable, but that's the most relative claim I could possibly make. Proud Mary, which employed the efforts three credited screenwriters, is absolute boilerplate: Mary (Henson) is the non-nonsense hitwoman for the African-American gang that controls one sector of Boston, and in the opening scene, she kills a man with blunt, humorless efficiency. Only then does she realise he had a preteen son (Winston), who failed to notice anything while playing a video game with a headset in his bedroom. One year later, Mary has found the boy, Danny, and is quietly following him around town, guarding him as he runs errands for Uncle (Xander Berkeley), a high-ranking member of the Russian gang (or it could be that they're Serbian; "untrustworthy Slavs" gets us close enough) that controls a different sector of Boston. Mary, you see, has a bad case of "I made you an orphan and now you're a drug-runner and possibly a sex slave, the movie is unpleasantly vague on that point" guilt.

Long story short, she kills Uncle in a fit of pique, and then finds that this had put her boss, Benny (Danny Glover, sleepwalking) in a terrible spot with Uncle's boss and nephew, Luka (Rade Šerbedžija), and a full-on gang war is likely to break out. But Mary doesn't particularly care about the charming sociopath Benny, or his glowering sociopath of a son, Tom (Billy Brown), an ex-lover who gets much too grabby with her still. She just wants to keep Danny safe at all costs, and if possible, escape Boston to make a new life with the boy.

That's pretty meat-and-potatoes narrative-making for this genre, and like any such thing could be good or bad based mostly on the execution; the execution, sadly, is utter dogshit. This is the fourth film directed by Babak Najafi, the second in English, and it's his immediate follow-up to 2016's London Has Fallen, an incredibly terrible film in every possible way. Even so, I took that as a comfort: it meant there was a basement. "The worst-case scenario will be that it's just as bad as London Has Fallen, but with Taraji P. Henson instead of Gerard Butler, and that's obviously a trade-up", I kept thinking to myself. Well, it is, and Henson is clearly having a good time as a taciturn killer with a heart of, at least, gold-plated steel. But it turns out there was a worse-case scenario than I had expected, and it's this dreary grind through plot points that are predictable in the smallest details from the instant we meet the characters, all of whom other than Mary herself are played by visibly bored performers (and maybe I should raise a glass to Margaret Avery, showing up to be all warm smiles in a single scene that appears to have been spliced in from a different, probably much stranger movie). The action is busy enough to be momentarily distracting from how soporific the rest of the film is - it is not a fast 89 minutes, and how grateful I am that it didn't try to be any longer - but given how the action tends to reliably overlap with unreasonably bad sound mixing that makes those soul classics pop up and down on the soundtrack like a bratty child was flipping a volume knob back and forth just to be annoying. And it's never more objectionable than the clumsy, inelegant way that the film uses Tina Turner's cover of "Proud Mary" itself. And if a film called Proud Mary can't even do a good job of showcasing that song, what possible fucking use could it be?