Will Ferrell, left, and John C. Reilly play two grown men who act like kids in the comedy Step Brothers. (Columbia Pictures)

After the box-office success of Talladega Nights, which starred Will Ferrell as a NASCAR driver and John C. Reilly as his self-sacrificing buddy, a re-teaming of Ferrell and Reilly was a no-brainer. Well, no brains is what we get in Step Brothers, a cheerfully inane vehicle in which the duo sends up Judd Apatow's man-child comedies. (Actually, it may qualify as self-parody since the ubiquitous Apatow produced this movie, too.)

If you thought the heroes of Forgetting Sarah Marshall, Knocked Up and The 40-Year-Old Virgin were immature, they’re nothing next to the overgrown boys played here by Reilly and Ferrell. Brennan (Ferrell) and Dale (Reilly) are two unemployed, unmarried men on the cusp of 40 whose development appears to have been arrested at 12. They whine and sulk. They won’t let anyone play with their toys. If they aren’t up in a tree house ogling a stash of skin mags, they’re in the garage trying out their Bruce Lee moves.

When we first meet them, both of these slackers are still living at home, each with a doting single parent who obviously hasn’t made much effort to nudge them out of the nest. But then Brennan’s mother, Nancy (Mary Steenburgen), meets, marries and moves in with Dale’s father, Robert (Richard Jenkins). (All in the opening credits, mind you.) Suddenly, the two spoiled brats are step brothers, forced to share a bedroom and their parents’ affections. Well, it’s either that, or the unthinkable: move out and get jobs.

At first, Dale and Brennan fight tooth and nail (and testicles, this being another genitalia-obsessed Apatow flick). Eventually, however, they make up and join forces against a common enemy: Brennan’s ultra-successful kid brother, Derek (Adam Scott), who has a wife, kids and not one but two careers. A shellac-haired smoothie, Derek belittles Brennan while unaccountably charming Dale’s dad with his unctuous talk.

Before long, Brennan and Dale are getting along like best buddies, bonding over such shared enthusiasms as dinosaurs, night-vision goggles and John Stamos. Then Nancy and Robert drop a bombshell. It seems the strain of having two hulking man-children under one roof is too much: the couple are getting a divorce. Will the boys be able to bring their parents back together? Will it involve growing up and finding gainful employment?

What follows is like some perverse mash-up of the Matthew McConaughey rom-com Failure to Launch and Disney’s The Parent Trap, with faint echoes of another Disney classic, Freaky Friday. Maybe there hasn’t been a body switch here, but you’d swear there were a pair of foul-mouthed preteens trapped inside Dale and Brennan’s hairy, paunchy frames.

This freaky element provides Step Brothers with its funniest moments. Ferrell and Reilly are comedians on whose faces the inner child is writ large, and they play their infantile characters with gusto. There’s a priceless scene early on when Ferrell’s sensitive Brennan, insulted by nasty Dale, sits at the family dinner table, his lower lip trembling, trying hard not to cry. As Dale, Reilly has the mercurial emotions of a kid whose hormones are just kicking in. One minute, his grumpy frown is burning a hole in the screen; the next, he’s squirrelly with excitement.

Dale (Reilly) and Brennan (Ferrell) get hassled by neighbourhood children in Step Brothers. (Columbia Pictures)

The two actors are good for each other. Although he proved he could carry a movie with the biopic spoof Walk Hard, Reilly is still a second banana by nature. His aggressive Dale is the perfect foil for Ferrell’s vulnerable Brennan, and he tackles the role with infectious exuberance. Ferrell, meanwhile, delivers one of his more focused performances as mama’s boy Brennan, an aspiring singer with an easily shattered ego.

Still, it’s all they can do to keep afloat a one-joke comedy that, even at 95 minutes, feels laboured. Step Brothers reunites Ferrell with director and co-writer Adam McKay, his collaborator on Talladega Nights and Anchorman, but the inspired weirdness of their earlier films is missing. They either recycle ancient gags (see the bunk-bed scene in the movie’s trailer) or set up potentially amusing sequences and then fail to exploit them. The slapstick violence is ineptly staged, and even for a movie with a 12-year-old mindset, there are one too many caca jokes.

Of course, Talladega Nights also had the inimitable Sacha Baron Cohen to give it an added jolt of lunacy; there’s no comparable co-star here. There is, however, a deliciously oily turn from Scott – the baby-shy husband of HBO’s Tell Me You Love Me. His weasel-faced Derek – an obnoxious martinet who forces his family to sing four-part harmonies – looks like a sly parody of Tom Cruise.

Jenkins (the dead dad of Six Feet Under) works up a head of comic steam as the increasingly apoplectic Robert. A cool Steenburgen, in typical Apatow fashion, plays straight woman to the men, spending most of her screen time smiling indulgently. Kathryn Hahn, in contrast, claws the scenery as Derek’s desperate wife – so desperate, she lusts after hapless, virginal Dale. Talk about your adolescent MILF fantasies. But the funniest female is Gillian Vigman as a potential employer interviewing Dale and Brennan — her blue eyes nearly pop out of her head in exasperation.

You have to credit the Apatow films for knowing how to have their cake and eat it. Step Brothers mocks the man-child while at the same time pandering to his juvenile tastes. The only misstep here was allowing the movie to receive an R-rating, thus depriving it of the one audience that can truly appreciate Dale and Brennan – the one that hasn’t yet hit puberty.

Step Brothers opens across Canada on July 25.

Martin Morrow writes about the arts for CBCNews.ca.