What if Jason Bourne hadn't been a waterlogged amnesiac? What if instead he had simply been too stoned to remember his dark past? It's a cute idea, and "American Ultra" almost makes a go of it. But the movie can't decide whether it wants to be a full-on action flick or a goofball comedy, and it never finds a unifying tone.

Jesse Eisenberg, master of quirks and fidgets, plays Mike Howell, a small-town pothead and 'shroom enthusiast living in slacker contentment with his girlfriend, Phoebe (Kristen Stewart). Mike works in a local convenience store, passing the wee hours rolling joints and doodling cartoons for a vaguely projected graphic novel. One night, a woman in a trenchcoat and shades walks in with a strange message. "Echo Choir has been breached," she whispers. "We are fielding the ball." Baked as usual, Mike asks, "Is that a lyric from something?" Turns out it's not.

Here the movie — which started out on a rom-com note with Mike and Phoebe at home — suddenly turns into a bloody action-fest. Standing outside the store spooning up ramen noodles from a cup, Mike is attacked by two men he previously spotted loitering near his car. To his own surprise, he whirls right into ninja battle mode, expertly deploying the only weapon at hand to take down his attackers. On the phone to Phoebe, he shouts, "I spooned this guy!" Then his car blows up.

Soon we learn that the trenchcoat woman, whose name is Victoria (Connie Britton), is a CIA agent who once ran a secret program to create super-soldiers out of social misfits like Mike. The program was ultimately shut down, but Mike remains unwittingly on call in his tiny hometown. Now Victoria's rival, an overwound CIA nut case named Yates (Topher Grace), is intent on terminating this last vestige of her old program in order to launch a new one of his own, staffed with homicidal psychopaths. (Chief among these are a scary machine-gun chick, played by Monique Ganderton, and a cackling loon called the Laugher, played by Walton Goggins, who's always fun to have on hand.)

Eisenberg and Stewart still have the easy chemistry they brought to the 2009 film "Adventureland" (although Eisenberg is too attentive an actor to be completely convincing as a guy in a constant pot fog). And Grace is pretty funny as a sizzling megalomaniac. There's also a neat psychedelic fight in a black-lit disco party room, and there are a couple of amusing action gags. (One involves a bullet and a frying pan.) But with so many guns, knives, hammers and grenades being brought into play, the attendant blood flow muddles the humor. Equally unhelpful in this regard is John Leguizamo's off-the-rails performance as a babbling white B-boy drug dealer, which has the unfortunate effect of recalling James Franco's similarly annoying turn in "Spring Breakers."

The story never gels, but the movie just keeps going. We're never in much doubt about where it's headed, but even with a 95-minute runtime, it takes too long to get there.

Kurt Loder is the film critic for Reason Online. To find out more about Kurt Loder and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com.