A high-concept action thriller about a past-his-prime hitman who finds himself targeted by his own mini-me clone, Gemini Man was actually conceived a couple of decades ago. Mel Gibson, Harrison Ford, Tom Cruise, Nicolas Cage and Brad Pitt were just some of the names attached to the project before it eventually ended up in the hands of Life of Pi director Ang Lee and star Will Smith.

Smith plays Henry Brogan, a 51-year-old assassin who has finally developed a conscience after killing 70-odd people for the DIA. He wants to call it a day, but the powers that be don't want Henry to sail off into the sunset with a gold watch – they'd rather his retirement was a little more permanent.

After a failed attempt to eliminate him, a new approach is needed, and baddie businessman Clay Varris (Clive Owen) steps up with a solution. It seems he handily crafted a clone of Henry more than 20 years ago, and Junior (a digitally de-aged Smith) is now all grown up and a trained assassin himself. Who better to kill Henry than someone who can match his every move?

With Mary Elizabeth Winstead as a fellow DIA agent on the run with Henry (who, thankfully, is pretty kick-ass and never a possible love interest) and Benedict Wong as Henry's pal, this had the potential to be a slick, fun actioner. Smith certainly deserves one – his last decent action movie was arguably 2012's Men in Black III – but Gemini Man never quite delivers.

The first big problem is the script, which doesn't develop any of the characters to the point that we either care about, or love to hate them. It's not really explained why Henry has suddenly decided killing people for money is a Bad Thing, nor why the DIA feel they have to murder him rather than just let him quietly retire. Clay's motives aren't much clearer, and the laughably brief explanation of how he cloned Henry in a lab sounds less complicated than one of the technical challenges on The Great British Bake Off.

While the 'young' version of Smith is pretty cool – most scenes where the tech is employed are darkly lit and the effect only looks ropey in a final scene shot in broad daylight – and the sight of watching Will Smith punch Will Smith is amusing, the overall appearance of the film falls short. Lee decided to shoot at an extra-high frame rate to give the action a realistic look, but the end result seems bizarrely cheap, looking more like a 1980s episode of The Bill than a mega-budget blockbuster.

Despite this, Smith is as watchable as ever, and fans will enjoy getting two of him for the price of one. But even he can't breathe life into a thinly sketched hitman from an ages-old screenplay; in the years since Gemini Man was originally written, we've been treated to many more nuanced, interesting screen assassins, from Bourne to Villanelle. Perhaps there is a lesson to be learned here, if nothing else: if a script has been gathering dust on a shelf for over 20 years, it's probably best to leave it there.

General release from Fri 11 Oct.