“Gemini Man” opens onboard a high-speed train rolling through the countryside in Belgium. I happen to have been on a train just like that a few weeks ago, and was amazed at how smooth it was even though we were traveling at 185mph.

Inside the movie’s train is a guy we’re supposed to assume is a bad man, since Will Smith is set up on a hillside a couple thousand yards away with a high-powered sniper’s rifle. Right away, we’re asked to suspend disbelief and buy into his talent as an assassin and, of course, he makes the shot.

I don’t mind giving away that little detail, because it’s what gets the ball rolling on an adventure that plays out over the next 115 minutes. I’m also going to tell you about a much larger detail of “Gemini Man” that has already been spoiled by the movie’s trailer and commercials.

When Smith finds out that the guy on the train may have been a target for the wrong reasons, he immediately becomes the object of a kill order given by the higher-ups in his own agency. If that sounds a little Bourne-like, it’s not a coincidence. When a regular group of special ops forces can’t kill Smith, his boss sends in a secret weapon — and here’s the spoiler I told you about! — who looks and sounds like a younger version of Smith. Thus, the dilemma: which Smith is a better killing machine?

Though the younger Smith is supposed to be about 23, he doesn’t look the way he did when he starred on TV as the “Fresh Prince Of Bel-Air.” That’s because director Ang Lee didn’t use footage from those days. Instead, the effect was created via motion-capture technology combined with the same de-aging CGI that made Samuel L. Jackson look so much younger in “Captain Marvel.”

The supporting cast includes Clive Owen as Smith’s villainous boss. Mary Elizabeth Winstead (who I liked in the “Fargo” TV series and the movie “The Hollars” and “Sky High”) plays a female agent surveilling Smith who quickly becomes his ally — but I’m happy to say she does not sleep with either of the Will Smiths.

From “Life Of Pi” to “Brokeback Mountain,” Lee’s movies have never done anything for me.  In fact, my review of his 2016 disaster, “Billy Lynn’s Long Halftime Walk,” is remembered by a lot of readers as one my most vicious — and it deserved every word. But in “Gemini Man,” he uses technology to his advantage, and is lucky to have such a charismatic star doing double duty.

Will Smith plus action movie used to equal huge box office, and the new version of “Aladdin” earlier this year certainly was helped by his presence. But I’d have to go back to “Concussion” and “Focus” in 2015 to find one of his star vehicles that I enjoyed. “Gemini Man” will probably open to about $25 million this weekend (which won’t displace “Joker” at the top of the box office charts), but whether word of mouth sustains it after that remains to be seen. That’s domestically. I bet it does a lot of business in China, where much of its financing came from, which explains the casting of Benedict Wong as one of Smith’s old Marine buddies.

For its mix of action and science fiction, and the opportunity to see two Will Smiths for the price of one, I give “Gemini Man” a 6 out of 10.