The latest collaboration in the Will Ferrell/Adam McKay/Judd Apatow partnership, "Step Brothers," adheres to the same axiom that powered their two previous features, "Anchorman" and "Talladega Nights."

It’s simple. By and large, people will laugh at grown men acting like 12-year-olds.

This movie takes that concept to its logical extreme: what would happen if two 40-year old men (Ferrell and John C. Reilly) who live with their parents (and the most extreme case of arrested development ever) became step brothers?

This isn’t Shakespeare.

It isn’t even playing at the same level of Apatow’s directorial efforts ("Knocked Up" and "The Forty-Year Old Virgin"), which skillfully blend extreme raunch and silliness with genuine heart. "Step Brothers" is, for the most part, completely content just to make you laugh with the absurdity of its premise. Heart and relevance need not apply.

But god help me, I laughed. I laughed a lot. This is not a perfect comedy -- "Step Brothers" is about ten minutes too long, and of the McKay/Ferrell/Apatow union, "Anchorman" still remains their masterpiece -- but when it’s funny, it’s very, very funny.

And despite the lunacy on display, McKay and Co. are up to something very canny. Adam McKay is an interesting guy. He may not be as heartfelt and honest a director of comedy as Apatow is, but he’s singularly gifted at making comedies that appeal to mass audiences.

The same way "Talladega Nights" worked for both the NASCAR and the anti-NASCAR crowds, so will different people take what they want from this movie. Some will sympathize with the struggle of Richard Jenkins and Mary Steenburgen as they try to manage their sons. Some will be all about the abundant, gleeful vulgarity. And some will get their kicks from the surreal references and bizarre, improvisatory, throwaway jokes.

Here’s a quick test for this last group: if the idea of Ferrell earnestly singing along to Andrea Bocelli doesn’t make you laugh, then this type of humor is not for you.

That’s not a problem, however, because there’s something for everyone, and in McKay’s hands it doesn’t feel like pandering. Rather, it’s a blending of comic styles, and it’s mostly seamless. This is an even bigger achievement when you consider the amount of improv that goes into making a movie like this (a reported six hours for one take); my hat goes off to McKay and editor Brent White’s ability to shape all that into a narrative.

Not that there’s much narrative to speak of. What plot exists is a thin frame on which to hang Ferrell and Reilly’s antics, both of whom are splendid.

Some may complain that Ferrell’s just up to his old shtick, and they’d be right, but so what? No one ever got on the Marx Brothers for coasting on their established comic personae. Ferrell’s a funny guy and he’s hysterical here.

And Reilly’s a wonder; who’d have thought that out of all Ferrell’s comic sidekicks (a list that includes Vince Vaughn, Paul Rudd and Steve Carell), Reilly, one of the best dramatic actors working today, would be the funniest and most inventive?

In a movie overstuffed with midnight destructive sleepwalking, live burials, and some of the funniest child abuse you’ll ever see, it’s the riffing between these two that you’ll remember -- the reason any of this nonsense works is completely owed to their superb chemistry and comic timing.

After a while, you half-believe what goes on in this movie could actually happen, these guys are such adept students of human behavior. Ferrell’s the quintessential momma’s boy, his blank face and trembling, prehensile lip suggesting both extreme sensitivity and extreme brain damage, while Reilly’s full, sloping brow and accusing bushy eyebrows immediately mark him as the more aggressive father-raised son -- he radiates unearned confidence and entitled confusion, sometimes all at once.

Plus, the two are still a little doughy, a little soft, their weight a physical manifestation of their parents’ coddling. Ferrell and Reilly’s dynamic, to one another, to their parents, has a kind of crazed, twisted logic to it, and it’s to the actors’ credit any logic exists.

This being a comedy in the Apatowniverse, Ferrell and Reilly aren’t the only ones delivering the funny. Rob Riggle makes a brief, yet memorable, appearance, and Kathryn Hahn is brutally funny as Ferrell’s manic depressive and extremely libidinous sister-in-law.

At the head of the class is Adam Scott as Ferrell’s obnoxious brother Derek. Chances are, you’ve seen him before but didn’t really notice him; he was Leonardo DiCaprio’s press agent in "The Aviator," and he plays Palek on "Tell Me You Love Me." If there’s any justice, this movie will make him a star.

Derek is so wonderfully loathsome you can’t take your eyes off him (his acapella rendition of "Sweet Child of Mine" is the highlight of the flick), and what compounds that is that his performance is a (very) thinly veiled take on Tom Cruise -- he’s all boundless (and irritating) enthusiasm and hyper-controlling charm. Scott steals every scene he’s in, and with this cast, that’s no easy task.

Where the flick stumbles the most is in the handling of Ferrell and Reilly’s parents.

Jenkins and Steenburgen are good, and that’s the problem. They play their worries and marital problems 100 percent real, and it doesn’t coalesce with the insanity surrounding them. I appreciate their efforts to bring depth to the movie, but it still doesn’t fit, and when they are asked to go really broad, the two seem even more out-of-place. Jenkins, in particular, suffers from this -- his T-Rex speech late in the movie feels like it’d be ten times funnier coming from anyone other than him.

In any other movie, this tonal imbalance might be more crippling. Not so here -- the funny stuff is so good it makes up for many of the movie’s missteps.

This is easily the funniest movie playing in theaters now, and it’s a welcome respite from the (admittedly brilliant) doom and gloom of something like "The Dark Knight." Between this and the underrated "Forgetting Sarah Marshall," Judd Apatow’s quickly becoming the Pixar of comedy; everything he touches (minus "Drillbit Taylor") is about ten times funnier and better made than other comedies out there.

It’s a very, very good time to like to laugh.   

Josh Katz is a freelance movie reviewer. He's been a movie fan since birth (much to the chagrin of his friends and family), and his top three favorite flicks are "Goodfellas," "Do the Right Thing," and "The Treasure of the Sierra Madre."