In case you didn't deduce it from the title, the film concerns two stumblebums who decide to impersonate policemen for kicks. They are 30-year-olds Ryan (Johnson), a former college athlete and mostly unemployed actor who spends his time barging in on children's football games, and Justin (Wayans), a struggling video game designer.
When the pair don police uniforms, borrowed from Justin's failed attempt at pitching a cop-themed video game, to attend a masquerade party that they mistakenly assume is a costume party, they find themselves the recipients of adoring female attention and respectful deference from everyone with whom they come into contact. Since the film is set in Los Angeles, that's the first clue that nothing onscreen will be remotely believable.
The pair is soon engaged in various hijinks that escalate when Ryan buys a used police car on eBay and outfits it with the literal bells and whistles. Although the milquetoast Justin is initially resistant to the charade, he enthusiastically joins in after finally getting somewhere with Josie (Nina Dobrev), the beautiful coffee shop waitress for whom he's been pining.
They eventually run afoul of a gang of Russian mobsters headed by the psychopathically violent Mossi (a buffed-up James D'Arcy). With the aid of a real cop (Rob Riggle) who at first mistakes them for the real thing, they get embroiled in a dangerous investigation in which they're way over their heads.
The screenplay, co-written by Nicholas Thomas and director Luke Greenfield, fails to mine the potentially humorous premise for the necessary laughs, with nearly all of the gags falling thuddingly flat. As if realizing that the humor wasn't going anywhere, the film suddenly lurches into action movie territory, complete with the sort of violence and brutality that is tonally at odds with the farcical setup.
While they demonstrate a genuine onscreen chemistry, the lead performers are thoroughly adrift, with Wayans mainly displaying pained expressions throughout and Johnson devolving into unfunny boorish mode.
The supporting players are equally ineffective at overcoming the lame material, with Andy Garcia barely making an impression (but picking up an easy paycheck) as an underworld heavy. The sole exception is Keegan-Michael Key, of the sketch comedy duo Key & Peele, who garners some laughs as a wildly braided street informant.
The end credits are accompanied not by the usual outtakes but rather by brief comic scenes that didn't make into the final product. Perversely, they're funnier than anything that's preceded them.
''Let's Be Cops,'' a 20th Century Fox release, is rated R for '' language including sexual references, some graphic nudity, violence and drug use.'' Running time: 104 minutes.
MPAA rating definition for R: Restricted. Under 17 requires accompanying parent or adult guardian.