Of course, Will Ferrell’s character in “Step Brothers” is a man who acts like an overgrown child — but this time, that’s literally the plot. In Adam McKay’s latest parade of absurdist buffoonery, Mr. Ferrell and acolyte John C. Reilly play newly made siblings with a common history of arrested development. But amid the milk-horking guffaws is the nervous giggle that the premise might be getting a little too close to the truth.

Not that an ounce of hesitation is detectable in Messrs. Ferrell and Reilly, who, as stepbrothers Brennan Huff and Dale Doback, fight, sulk, exult in bunk beds, babble while sleepwalking, and trigger a weirdly gratuitous gag involving prosthetic testicles. Rather than being the belly-flopping centerpiece, they’re thankfully complemented by an ensemble including a scene-stealing Richard Jenkins as Dale’s doctor father and Mary Steenburgen as Brennan’s sweetly over-sympathetic mother.

The two single parents, blissful in their second-act romance, have moved in together with men-children in tow. Brennan and Dale, like any self-respecting 13-year-olds, or 13-year-olds in the bodies of almost-40-year-olds, circle each other warily at first. But aided by a shared loathing of Brennan’s brother Derek (Adam Scott), they bond and get back to the urgent business of karate in the garage and Hustler-stocked tree houses.

Pushed by Dr. Doback, they don tuxes for a job search of unrepentant incompetence (stunning even an interviewer played by Seth Rogen). They end up straining their parents’ marriage and hokey dream of a yacht vacation. Throughout, Messrs. Ferrell and Reilly slightly tweak the former’s patented hormonal chemistry of improbable machismo and Homerian idiocy (“Simpsons,” not “Iliad”) toward extended toddlerhood.

An equally strong voice in “Step Brothers” belongs to Mr. McKay, who co-wrote the screenplay with fellow storied “Saturday Night Live” alum Mr. Ferrell. His absurdist bent and improvisation-powered inventiveness (shared with Mr. Ferrell) lead to some fresh left-field punch lines and briskly insane bits of business, the sort that often perked up slow stretches of his previous directorial outings, “Anchorman” and “Talladega Nights.”

The downside is that “Step Brothers” can feel like a loose assembly of scenes and tilt toward what might be called the hyperbolic style: a gleeful dive into parodic vulgarity and aggressiveness that sometimes leaves you too bewildered to laugh. It suits high-earning über-jerk Derek, the second (or third) coming of yuppie-hood, but it can also get grating, and Derek’s repressed, obscenely horny wife (Kathryn Hahn) has to be seen to be believed as she attaches herself to babe-in-woods Dale.

At shriller moments the style places the movie not far from another summer comedy, “You Don’t Mess With the Zohan,” also co-written by an “SNL”-alum, the absurdist Robert Smigel (whose penchant for gonzo animation emerged in Zohan’s CGI-aided feats of impossible physical prowess). “Step Brothers” at least stays restless enough to keep from becoming dull in its final stretch, in which Brennan and Dale can be seen actually holding down respectable jobs.

Brennan’s comically emasculated manner at this point, not to mention Derek and his air-punching minions, lets you know where “Step Brothers” stands, emerging as essentially another romp in the name of guy friendship and all that is awesome. Not that there’s anything wrong with that, and Mr. Ferrell does benefit from not having to be the Chris Farley-esque center of attention (he even keeps his shirt on, mostly), while Mr. Reilly confirms that he works best with a partner in crime (cf his self-defeating biopic parody “Walk Hard”). I’m not saying they have to move out, but they might look for newer ways of exploiting the talent for never growing up.