I'll deny it if quoted, but "Tropic Thunder" -- a true cornucopia of bad taste -- is the funniest film of the summer so far.

How can you deny your own words in print? I don't know, but I'll find a way. Such is my guilt for laughing throughout Ben Stiller's outrageous action comedy about the most expensive Vietnam War film ever made. It's not just politically incorrect. It's politically inexcusable.

The joke (if you can take it) in this movie-within-a-movie is really on Hollywood, its profligate productions and pampered performers. At the outset, our unholy trinity of stars is established by funny faux trailers of their hits:

Tugg Speedman (Stiller) is an aging Rambo whose desperate longing for an Oscar led him to the title role in "Simple Jack," the touching tale of a mentally challenged farmhand who chats with animals.

Likewise looking for legitimacy is coked-up Jeff Portnoy (Jack Black), whose film franchise "The Fatties" consists entirely of fart jokes.

Brilliant Australian actor Kirk Lazarus (Robert Downey Jr.), on the other hand, has already been legitimized by winning five Oscars (one for his compelling portrayal of Neil Armstrong). His upcoming movie about 12th-century monks in forbidden love, co-starring Tobey Maguire, is eagerly awaited. Kirk is so into "the method" and miraculous physical makeovers that he had his skin surgically darkened in order to play black Sgt. Lincoln Osiris in the 'Nam film -- much to the annoyance of actual African-American Brandon T. Jackson as hip-hop star Alpa Chino (say it out loud) in a supporting role.

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"Tropic Thunder"

3 stars = Good
Ratings explained

• Starring: Ben Stiller, Robert Downey Jr., Nick Nolte, Jack Black, Brandon T. Jackson and Jay Baruchel.
• Rating: R for pervasive language, sexual references, violence and drug material.
• Web site: tropicthunder.com

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When out-of-control costs and the collective star egos threaten to shut down his bloated production, British director Damien Cockburn (Steve Coogan) conspires with memoirist Nick Nolte's John "Four Leaf" Tayback (on whose 'Nam exploits the film is based) to shanghai and deposit the stars deep in the Southeast Asian jungle, forcing them to shoot the film as a guerrilla reality show. But the director instantly eliminates himself by stepping on an old land mine, leaving the troupe to its own improvised explosive devices -- and an apocalyptic encounter with a 12-year-old drug lord, who happens to have (and love) a video of "Simple Jack."

The screenplay, by director Stiller and Justin Theroux, may remind you of "Delta Farce," about three bumbling Army reservists bound for Iraq who are accidentally dropped off in a Mexican village that they proceed to "liberate." It's also reminiscent at times of "M*A*S*H," but mostly it's a spoof of "Apocalypse Now," "Saving Private Ryan" and other big-budget war extravaganzas, complete with 'Nam-era Huey helicopter raids and massive napalm explosions.

Neither Stiller ("Night at the Museum") nor Black ("King Kong") does much for me, in general. But they're quite amusing here, as are Jackson and Nolte.

The one who can do no wrong in my book (as opposed to some law-enforcement books) is Downey, "inhabiting" his part here as well as he did Charlie Chaplin.

In a godsend of unexpected publicity, no fewer than 22 disability groups have called for a boycott of "Tropic Thunder" for its grotesque sendup of hokey films about the disabled and use of the word "retard." The epitome of its targets is the cloying "Charly" (1968), which brought Cliff Robertson an undeserved Oscar the same year that deserving winner Tony Curtis wasn't even nominated for "The Boston Strangler." One of Downey's funniest bits is his critique of Stiller's performance as Simple Jack: Actors who win Oscars playing the mentally challenged, like Hoffman in "Rain Man" or Hanks in "Forest Gump," "never go full retard."

You can see why advocates for the disabled would be offended. But I can see why Vietnam vets could have even more of a problem, perhaps not appreciating the hilarity of Stiller-Nolte's blown-off hands gag, and perhaps having more reason to take and register offense than the disabled.

My bet, if you go, is that you'll be most amused by a character named Les Grossman, an amalgamation of Scott Rudin, Jon Peters and every tyrannical studio chief who ever lived. His closed-circuit-video tirades of over-the-top scatological invective are contrasted by the gentle tune of his cell-phone ring: "Sometimes when we touch, the honesty's too much ..."

See if you can figure out the name of the star who plays him before the credits roll.