Ryan (Jake Johnson, left) and Justin (Damon Wayans Jr.) get some extracurricular use out of their LAPD Halloween costumes in “Let’s Be Cops.” (Frank Masi, SMPSP/Twentieth Century Fox)

Like a down-market “Sharknado” of buddy-policemen movies, “Let's Be Cops” seeks to capi­tal­ize on the success of the “21 Jump Street” franchise with lackadaisically exploitative relish: Jake Johnson and Damon Wayans Jr. play a couple of 30-year-old washed-ups living in Los Angeles who, for reasons far too convoluted and tiresome to go into here, possess two perfectly fitting LAPD uniforms. Walking home from the party they wear them to, they find that they’re getting female attention and respect for the first time since college, a frisson of self-worth that leads Ryan (Johnson), a former football hero, to take the impersonation as far as he possibly can.

Ryan’s best friend, Justin (Wayans), is more reticent, but he keeps up the ruse partly to impress a pretty waitress played by Nina Dobrev. Director and co-writer Luke Greenfield brings no flair or energy to a flimsy premise that, like a piece of gum he’s found on his shoe, only grows thinner and more annoying the longer he tries to stretch it out. Worse, he fails to capi­tal­ize on the chemistry between Johnson and Wayans, who have proven their mutually supportive mojo on the sitcom “New Girl.”

“Let’s Be Cops” is the kind of movie that depends for laughs on tired bits involving kids swearing, and sustains the audience’s interest with frequent excuses to ogle shapely women dancing provocatively in bars, at parties or, in one unsavory instance, on her own skankily disheveled couch. Johnson and Wayans are both gifted comic performers but are given way too little to do in a film that wends its way from set piece to set piece, not with antic glee but desultory and-then-this-happens randomness.

Things begin to look up when Rob Riggle arrives on the scene, looking like he just popped in from his hilarious cameo in “The Hangover.” But like the two leads, Riggle is quickly sucked into the “Let’s Be Cops” vortex of lazy plotting and uninspired, workmanlike execution. His first scene with the guys involves a hardware store break-in conducted, for no apparent reason other than to set up a cheap gross-out gag, by a nearly naked sumo wrestler.

After several protracted sequences featuring a slithery villain (James D’Arcy) and a tattooed, gold-grilled contraband runner (Keegan-Michael Key), “Let’s Be Cops” turns deadly serious for its final half-hour, with Ryan and Justin confronting real-life crime, corruption and gun violence and the filmmakers making an unconvincing point about the difference between video-game mayhem and the real thing. There’s nothing real about “Let’s Be Cops,” which, if there’s any justice in the world, should earn its perpetrators a healthy stretch in movie jail.

no stars

R. At area theaters. Contains profanity, including sexual references, graphic nudity, violence and drug use. 104 minutes.