If you've seen one Will Ferrell movie, you've got a good idea what to expect in every Will Ferrell movie: Ferrell will act like a dim-witted idiot and there will be various levels of dumb fun for everyone.

In "Step Brothers," the dumb fun is set at about medium level.

Ferrell teams again with "Talladega Nights" co-star John C. Reilly and shares writing credits with his favorite director, Adam McKay ("Anchorman," "Talladega Nights").

Reilly and Ferrell play a couple of 40-something layabouts who are thrown together when Mary Steenburgen, the mother of Ferrell's Brennan, and Richard Jenkins ("North Country"), the father of Reilly's Dale, meet, fall in love and marry.

The result, though, is hardly the "Brady Bunch." The new stepbrothers are both unemployed (for good reason), unintelligent and highly territorial. Thrown together in the same bedroom, friction is sure to happen.

"Step Brothers" doesn't really have a plot; it simply has one joke, which it rides for all it's worth. Early on, Dale and Brennan fight over everything, causing much mayhem. Later, they find that they have a lot in common ­-- particularly a hatred for Brennan's obnoxiously successful brother Derek (Adam Scott) -- and in joyous cooperation they cause more mayhem.

Jenkins and Steenburgen are both good in their limited roles as parents, and the supporting cast is generally good. Scott has some great moments, and Kathryn Hahn ("Crossing Jordan"), as Derek's unhappy wife, offers several laughs.

The film -- which earns its R rating primarily with a string of expletives -- is wildly uneven, however. There are some riotously funny moments, but these are outnumbered by the jokes that fall flat or don't go anywhere. "Step Brothers" has the feel of something the stars and the director sketched out over beers after filming "Talladega Nights," then ad-libbed long enough to get 90 minutes of film.

Ferrell and Reilly do make a pretty solid comic duo, with good timing, but infantile behavior can only take them so far.

Rental Recommendation: Ferrell's best role is still Ron Burgundy in "Anchorman." Grade: A-

ouellette1918@gmail.com