At best I was buying about half of what “American Assassin” was selling.

At. Best.

Michael Keaton as a battle-scarred, Cold War Secret Ops legend now living far off the grid in the obligatory remote cabin, where he trains a small band of elite young warriors for ultra-dangerous, top-secret overseas missions the USA will never even acknowledge? Sure. I’m not sure who goes out and gets the groceries during the whole training period, but all right.

Sanaa Lathan as Keaton’s supervisor, a tough-as-nails commander who has a soft side for Keaton because her late father was once his partner? OK.

But then we start sliding down a slippery slope into all sorts of booby traps, including but not limited to:

• Unwise casting choices in two key roles.

• Increasingly ludicrous plot developments — even for a slick, escapist thriller.

• Dubious science about the potency of a nuclear warhead.

• Intellectually lazy pop psychology, much of it heavy on the daddy issues, as character motivation.

• An extended and nasty torture porn sequence in which the recipient apparently believes that on some level he deserves the brutal punishment inflicted on his person and he almost revels in it, e.g., after a fingernail is torn off, he grins maniacally like the Joker and screams, “I’ve got nine more!”

Dylan O’Brien (from “The Maze Runner” and MTV’s “Teen Wolf” series) plays Mitch Rapp (now there’s an action novel series hero’s name right up there with JASON BOURNE or JACK REACHER), a slender, seemingly passive, amiable grad student on holiday at a beautiful tropical resort with his lovely and sweet girlfriend.

We witness Mitch’s marriage proposal almost entirely through the lens of his phone. Come on Mitch, get in the moment and stop recording. Don’t be such a millennial.

Suffice to say things go tragically wrong just moments later. Director Michael Cuesta stages the first of multiple extended, impressively choreographed and quite violent action sequences. (Given the horrific and seemingly endless series of terrorist attacks around the world on innocent human beings in public places, it might not be everyone’s idea of entertainment to see a fictional terrorist attack carried out on the big screen with such undeniably shocking punch.)

Cut to a year and a half later.

The nice young man has disappeared. Mitch has noticeably bulked up and has transformed himself into a formidable fighting machine, skilled in the martial arts, knife throwing and other forms of combat. (Fast work!) He’s also become a notorious hothead, getting kicked out of a dojo for ultra-violent, unacceptable fighting techniques and scaring the life out of responsible gun owners at a shooting range. Mitch has enough chips on his shoulder to fill a can of Pringles.

Mitch’s apartment is a combination of gym and high-tech lair. Through brilliant cyber manipulations, he has managed to infiltrate the terrorist cell that ruined his life, to the point where the highest command believes his supposedly radical anti-American rants and is ready to bring him onboard.

Meanwhile, CIA Deputy Director Irene Kennedy (Lathan) and her team are keeping a close watch on Mitch, trying to determine whether he’s become a homegrown terrorist or he’s actually going into the belly of the beast to cut off the beast’s head on what would surely be a suicide mission.

Irene eventually takes a crazy chance on Mitch and sends him to the aforementioned Cold War legend Stan Hurley (Keaton) for intense training. Hurley does everything he can to break the lad and send him home crying, but that darn Mitch won’t give up.

With the world reeling from a series of terrorist attacks on military and civilian targets seemingly designed to start a nuclear war in the Middle East,  Irene enlists Stan and a handful of Stan’s best recruits (including Mitch) to hunt down the operative masterminding the madness. They team up with a deadly, mysterious, possibly duplicitous and most definitely glamorous Turkish agent (Shiva Negar), who of course has to pose as Mitch’s lover at one point as part of the undercover operation.

Of all the insanely implausible developments in “American Assassin” — and there are many — some of the most outrageous involve that Turkish agent. It’s as if she wandered into this putatively gritty thriller from one of the more cartoonish James Bond movies.

Then there’s “Ghost,” once of the U.S. Navy and a former protégé of Stan’s, now a rogue terrorist intent on creating a nuclear war, mostly because his feelings were really, REALLY bruised by father figure Stan back in the day. He’s become an unhinged madman who talks far too much and delights in torturing his nemesis (again, the James Bond playbook) instead of just shooting the guy in the head and continuing on with his evil mission.

Taylor Kitsch, a solid actor who has carved out a respectable feature career with supporting roles in films such as “Savages” and “Lone Survivor” and a stint on Season Two of “True Detective” following his breakout role on the TV series “Friday Night Lights,” plays Ghost.

Sorry, but no sale. I didn’t buy Kitsch as a deranged psychopath willing to sacrifice thousands of lives and throw the world into chaos any more than I bought O’Brien as the most lethal human weapon in the United States’ anti-terrorism arsenal. Sometimes acting comes across more like pretending.

★★

CBS Films and Lionsgate present a film directed by Michael Cuestra and written by  Stephen Schiff, Michael Finch, Edward Zwick and Marshall Herskovitz, based on a novel by Vince Flynn. Rated R (for strong violence throughout, some torture, language and brief nudity). Running time: 112 minutes. Opens Friday at local theaters.