Once upon a time, when a screenplay required a character to appear at two very different ages, directors cast two actors in the parts, hoping that the audience would accept the ploy. Sometimes the convention worked, sometimes not. Nowadays, with digital trickery all the rage, artificial de-aging is often used instead—sometimes successfully (the best job is probably still that done on Michael Douglas in the “Ant-Man” movies) but more often not (see Samuel L. Jackson in “Captain Marvel”).

Ang Lee has shown himself a devotee of technical advances; he reveled in them so much in “Life of Pi” and “Billy Lynn’s Long Halftime Walk” that they pretty much overwhelmed the final result, to the pictures’ detriment. It’s not surprising that he’s gone full-bore with the de-aging process in an attempt to make something of “Gemini Man,” a decades-old screenplay that even in the 1990s, when it was first confected, must have seemed a particularly limp example of action-movie pulp. He and his script doctors have tried to spruce up the thing, but it remains a numbskull affair, poorly plotted and weighed down by dumb dialogue and flat characters, and the twenty-first century effects are frankly not good enough to justify it.

The story idea was first put forward in 1997 by Darren Lemke, who remains one of the three credited screenwriters, at a time when the successful cloning of Dolly the Sheep, announced in 1996, created a stir about the possibility of human duplication. Instead of treating that scientific miracle in even vaguely serious terms, Lemke concocted a plot about an aging sharpshooter who was targeted to be killed himself by his clone as part of a government conspiracy. His pitch was bought by Disney but though it passed through the hands of many directors and stars, never made it into production—supposedly because the requisite optical effects had not progressed far enough.

One hopes, however, that it was really script deficiencies that obstructed the project for so long, because even as spiffed up by David Benioff and Billy Ray, the narrative is less convoluted than messy, full of plot holes and nonsensicality.

The heroic protagonist is Henry Brogan (Will Smith, showing occasional glimmers of his old charisma), a fifty-one-year-old assassin, the best marksman of them all, who’s been employed for years by the DIA, one letter up from the CIA and thus, presumably, even more lethal in its methods. Having chalked up seventy-two hits—all of evil people, one presumes—at the instance of his long-time contact Patterson (Ralph Brown) and Patterson’s boss Lassiter (Linda Emond), Henry is growing weary of the trade: he’s beginning to feel bad about offing folks, however much they deserve it. So when, working with a young agent named Moreno (E.J. Bonilla), he kills a guy he’s been told is a terrorist through the window of a speeding train (an impossible shot, of course), he decides to retire to his remote Georgia home to build birdhouses and go fishing.

Unfortunately, his minders have lied to him. He’s informed by an old pal (Douglas Hodge) that his supposedly last target was no terrorist, but rather a master biologist. That irks our man, who suddenly finds himself targeted by the DIA. Luckily he escapes with the aid of pretty agent Danny Zakarweski (Mary Elizabeth Winstead), and soon the duo is on the run from their would-be killers together, aided by another of Henry’s old Marine buddies, Baron (Benedict Wong, providing what little comedy relief there is here).

The ultimate villain is Brogan’s old corps commander, evil Clay Verris (Clive Owen, in full steely-eyed, nasty mode), who runs a mercenary-providing corporation called Gemini. He’s trying to engineer super-soldiers, and his prize specimen to date is the clone he made thirty years ago of Brogan himself, an even super-er kid he’s raised as his own son, Junior (the de-aged likeness of Smith). Apparently the combination of genes and tough training has worked, and when Junior confronts Henry first in Cartagena, where they engage in a spectacularly prolonged motorcycle chase, and then in Budapest, where Henry and Junior have at it mano-a-mano in an underground tunnel, the face-off is meant to be amazing, even if age does tell.

Of course Henry and Junior eventually bond, and everything culminates in a big confrontation with Verris and his private army. As if that were not enough, the picture then adds a wacky coda to the mix, involving a soldier even super-er than Junior. It makes for yet another action set-piece peopled not only by Smiths young and old (as well as plucky Winstead) but a shipload of other CGI effects.

This whole scenario, apart from the cloning element, is sadly reminiscent of that of the recent “Angel Has Fallen” (just think of the relationship between the characters played by Gerard Butler and Danny Huston). But adding the cloning business makes it even worse. The problem isn’t just that the fundamental premise of the movie is inherently simplistic about what the outcome of human cloning would be, and that the working-out of the idea is then executed with lots of stumbles and implausibility—Lee’s handling of the expository scenes throughout is halting and stiff, which exacerbates the silliness of much of the dialogue (a good example is a conversation in Budapest between Brogan and a Russian played by Ilia Volok)—but that the whole clone-vs.-original aspect of the story is poorly handled from a visual perspective.

Lee and his technical crew are pushing the envelope in this respect, of course, but frankly on the evidence of this film, Hollywood is not yet capable of replacing humans with computerized replicas. Junior looks not so much like a real younger version of Smith, but what he is—a simulation that isn’t quite right: he’s a glossy, artificial entity, whose lip movements in particular are off. The effect just hasn’t been perfected yet, and as you’re watching the movie that represents a persistent irritation, particularly noticeable in dialogue scenes.

But it’s also apparent in the action sequences, which are frankly disappointing—the motorcycle chase choppy, the fight in the tunnel a murky mess, and the big final confrontation almost impossible to follow, blazing beams of light obliterating much of what happens. It may be that here Lee’s choice of using the high-resolution, more-frames-per-second format he employed in “Long Walk” is a hindrance. The process he and cinematographer Dion Beebe have used gives greater clarity to the images—the backgrounds here are remarkably crisp and detailed, as are the close-ups of real actors—but it also makes the imperfections in the CGI-heavy sequences more obvious than they might otherwise appear.

Even apart from that, however. “Gemini Man” is one of those high-concept projects whose time has long passed, if it ever existed; even back in 1997, the story would have been dismissed as goofy (why, for instance, if Verris was intent on producing an army of super-soldiers, didn’t he clone hundreds or thousands of Brogans, and then train them all accordingly—after all, he had the necessary DNA?). Things could be worse, of course: the movie isn’t as bad as Smith’s recent big-budget action bomb, the Netflix atrocity “Bright.” Thankfully no one suggested that the young Brogan should be not computer-generated but played by Smith’s son Jaden, with whom he teamed so unimpressively in M. Night Shyamalan’s infamous “After Earth.” And Winstead remains a very appealing screen presence. But to say that a movie could have been worse is hardly a ringing endorsement.

At one point in “Gemini Man,” Verris describes a failed DIA attempt on Brogan’s life as “like watching the Hindenburg crash into the Titanic”—an apt description, one could argue, of “Gemini Man” itself. Incidentally, there was a television program of the same title—a 1976 one-season NBC sci-fi series about an agent (Ben Murphy) who could turn invisible by a click of his watch. At times during Lee’s elephantine miscalculation, a viewer might wish he could make that watch work on this literally misbegotten movie.