We've all experienced the horror of the guest who wouldn't leave.

But what if that malingerer were your own child?

We find out, in raunchy, raucous "Step Brothers," a movie about 40-year-old "boys" (Will Ferrell, John C. Reilly) forced to share a room when their parents (Mary Steenburgen, Richard Jenkins) marry.

On a superficial level, the movie is a one-joke comedy about arrested development - Will Ferrell squeezing into the same Pablo Cruise T-shirt he wore when he was 14, fighting with his new stepbrother about an off-limits drumset (infantalized man, of course, is right up Ferrell's alley).

But as "Step Brother" plays out, the expected comedy about perma-boys Ferrell and Reilly blends with a borderline horror-movie about the poor parents who are stuck with them.

The encumbered Jenkins and Steenburgen feel an instant bond but rarely broach the subject of their inert man-children. It's a funny idea: for parents in a world of "my child is an honor student" bumper-stickers, caring for an underachieving child has become the Love That Dare Not Speak Its Name.

Writer-director Adam McKay (Ferrell's collaborator in "Anchorman" and "Talladega Nights") plays the set-up for kooky laughs - in one bit, the fortysomething Ferrell and Reilly are attacked by neighborhood bullies who are only 12.

Steenburgen is there to provide Bactine and sympathy ("Didn't we talk about taking the long way home?"), but she's frustrated - she and Jenkins are ready to retire and take a long sailing trip but can't because the boys are helpless.

Jokes center on attempts by Jenkins/Steenburgen to place the boys in jobs and eventually evict them from the house. This turns into a black-comedy war that threatens to separate brothers and parents alike.

All builds to a wildly absurdist (in the McKay-Ferrell "Anchorman" tradition) and improbably funny conclusion that plays like a crazed version of "The Parent Trap" and climaxes with a memorable musical number.

There are "Mamma Mia!" fans who suspect I have something against the movie musical. Not true. I just didn't hear anything as good as Ferrell crooning "Con te Partiro." *

Produced by Judd Apatow and Jimmy Miller, directed by Adam McKay, written by McKay and Will Ferrell, music by John Brion, distributed by Columbia Pictures.