A pinch of "Jay and Silent Bob Strike Back," a dash of "Adventureland," and a dollop of "The Bourne Identity" serve to concoct "American Ultra," a genre-bending stoner comedy that proves most successful as a romantic action-thriller. The director of one of the decade's worst found-footage films—Nima Nourizadeh (2012's "Project X")—and the screenwriter of one of the best—Max Landis (2012's "Chronicle")—have joined forces for a film that is tonally all over the place but also quick and unhinged enough to amuse for an hour and a half. At first glance, Jesse Eisenberg (2015's "The End of the Tour") and Kristen Stewart (2015's "Clouds of Sils Maria") may not be one's imagined first choices to play characters who require they be more broadly acerbic than the usual roles in which they are cast, but they are game participants and exceedingly copacetic as a pair.

Mike Howell (Jesse Eisenberg) and Phoebe Larson (Kristen Stewart) are the perfect fucked-up couple ("She was perfect," says Mike in voiceover, "and I was fucked-up"), small-town West Virginians who get by on working-class jobs, ample marijuana, and each other. Mike wants to take their relationship to the next level and has secretly just bought an engagement ring, but he worries that his longstanding hang-ups and anxieties will continue to hold them back in their modest lives. When Mike is approached during his evening shift at the Cash-N-Carry by a mysterious woman, CIA agent Victoria Lasseter (Connie Britton), he is left completely perplexed by the coded message she is trying to send him. Before long, the town has been put into quarantine by government officials—the news talks of a cockamamie typhoid outbreak—and Mike and Phoebe are on the run from a seemingly nonstop arsenal of deadly assassins. As information he never realized he knew and sharp fighting skills he doesn't recall having used before come flooding back, Mike begins to suspect there is a lot about his past that he has yet to discover.

"American Ultra" is set in the fictional town of Liman, an undoubted reference to "The Bourne Identity" director Doug Liman that cannot possibly be mere coincidence. It is just one of many sly cinematic homages that range from "True Romance" to TV series "Chuck" in a film that is unsuspectingly sweet in the face of blood being copiously strewn across the screen and protagonists Mike and Phoebe getting progressively banged-up as their night from hell comes to a head. The pot-related humor is a fairly minor part of the story, and it is just as well since this is one of its most throwaway aspects. Where the picture best engages is in its go-for-broke insanity and mounting chaos as Mike and Phoebe's sleepy little burg becomes overrun by nefarious, gun-toting figures and faux-researchers in Hazmat suits. Mike is at a loss for what is going on, and as he fights to survive while sparking up another doobie, his paranoia gives the action-packed, intentionally sketchy proceedings a more enthralling sense of intrigue.

A stringy-haired, dazed, strikingly agile Jesse Eisenberg and a loose, endearing Kristen Stewart anchor the outlandish goings-on with a down-to-earth identifiableness as Mike and Phoebe. A little bit more development of their relationship early on would have been welcome, but what is here is still enough to like them and later care about them when interpersonal complications arise. Both actors came to the project ready to have a good time, and it shows in their spirited performances. Connie Britton (2015's "Me and Earl and the Dying Girl") is a standout in the juicy part of empathetic CIA agent Victoria Lasseter, who opts to go against rotten superior Adrian Yates (Topher Grace) and warn Mike of his imminent danger. In what is a sly subliminal plot point, Victoria's tight bun of hair gradually, with each new scene, comes lustrously down, mirroring her character's brave decision to stand up for herself and fight for what she knows is right. Also making an impression, Topher Grace (2011's "Take Me Home Tonight") is great at playing utterly contemptible as power-hungry agency head Adrian Yates, while Tony Hale (2013's "The Heat") is beguilingly high-strung as CIA desk jockey Petey Douglas, torn between his boss' wrongheaded demands and colleague Victoria's more honorable plight. A brief moment where Petey gets a phone call from his same-sex beau helps to give his character a life outside of his job while refreshingly not calling attention to itself, and that's how it should be.

"American Ultra" takes itself mostly seriously but is pure goofiness all the same. The general concept has been done many times before, but director Nima Nourizadeh gives it an offbeat indie flavor and a welcome romantic undercurrent. While there are a handful of lightly funny moments sprinkled throughout, the picture is on sturdier footing when it comes to the action and combat set-pieces. A climactic showdown in a home goods store is especially impressive, much of it playing out in an extended, rousingly complex single shot. Normally, the liberal loss of life being depicted would come off as distasteful and short-sighted, but its uber-gritty, all-wild conception somehow works within the confines of its over-the-top story. As bad guys, authority figures and the occasional bystanders meet violently ill fates, the film, at its core, is about a guy who loves a girl and longs to let her know he wants to spend the rest of his life with her. In between the blistering gunfire and spoon-stabbings to the neck, "American Ultra" isn't without its oddball charms.