How many foes must a man mow down before you call him a man? That’s the question at the center of American Ultra, the latest twist on an action-film subset of unexpected badassery.

Jesse Eisenberg is Mike, an existentially adrift convenience-store clerk and champion stoner in small-town West Virginia who one night discovers he is an extremely efficient killer. His quick-thinking skills with cup ramen, spoons and kettle bells, among other improvisational weapons, make Mike a murderous MacGyver. Or, given his craving for the chronic, a Manchurian Candidate with the munchies.

As Eisenberg switches in a millisecond from little boy lost to limb-snapping lug, the actor’s twitchy timidity, cellophane confidence and meek physical stature are the obvious jokes. He nevertheless sells them anew every time through immaculate who-me? body language, leaving Mike unnerved and bemused by his butchery once the adrenaline subsides and muscles relax. But just as Mike does for the comic-book characters he creates during down time at work (which would seem to be all eight hours of every shift), Eisenberg expertly shades the sketches of Mike’s psyche. In one early pot-haze moment, he contorts Mike’s funny, far-out-man fixation on a tree into devastating, melancholy introspection.

Regardless of his panic, Mike sets about carving up bad guys sent to off him by snotty CIA functionary Yates (Topher Grace, so apoplectically aggressive he seems constantly on the verge of going up). And maybe he can carve out a more promising future with put-upon lady love, Phoebe (Kristen Stewart).

Last week, Ultra screenwriter Max Landis (son of filmmaker John Landis and co-writer of 2012’s excellent found-footage superhero film Chronicle) tweeted: “WARNING: @americanultra reviews come out next week. American Ultra is a movie with TWISTS, so don’t read reviews if you don’t want SPOILERS!”

Almost nothing you’ve read above reveals anything more than the ads. Are there twists? Yes. Can I write around them? Absolutely. After a promising start, so does Landis’s script … in frustrating circles that diminish American Ultra into a bloody boondoggle.

It’s not for a lack of ambition, as Landis tries to establish the vibrant, violent verisimilitude of a graphic novel in an onscreen original. The persuasively ghoulish makeup that renders Eisenberg a walking contusion radiates the pulpy shade of purple you’d find on the pages of a comic, and Landis has fun with how Mike’s stoner paranoia bears itself out; the film’s very title is a clever conspiracy theorist in-joke.

The problem is all of the character-arc shortcuts Landis takes solely to move Ultra quickly from one brawl to the next. It’s an impatient unraveling of consistently strong work from Eisenberg and Stewart, who rekindle their sweet sparks from 2009’s gem Adventureland to prop up the notion of eternal soulmate bonding missing from the script. In the end, that thirst for Ultraviolence strikes a lingering sour note. Again, no spoilers, but the “frothy” finale feels awfully fatalistic — a fundamental misread of empowerment as Mike and Phoebe would define it.

Although staged with verve by director Nima Nourizadeh (2012’s Project X), the fights eventually lose their sting, the jokes become increasingly witless and the storytelling grows more artless. Instead of caring whether Mike and Phoebe (or their relationship) will make it, you’ll wonder why, instead of laying waste to a town or leaving his best guys sitting in the back of a truck, Yates never tries something stealthier to kill Mike like, say, a batch of bad weed.

Ultra also leaves hanging a trio of great character actors — Connie Britton, John Leguizamo and Tony Hale — and criminally reduces Walton Goggins (Justified), an actor of such prolific and persuasive verbal menace, to a goofy Mike Tyson-esque lisp.

Aided by martial arts training (footage of which would make the greatest Blu-ray extra ever), Eisenberg gives us a credible action hero as spiritually wounded as he is physically pummeled. But he can only keep American Ultra moving so long before it tops out at modest velocity, right down the middle of the road.