The show actually starts earlier than you realize. Before the DreamWorks logo even appears on screen, we see a fake commercial and three brilliant spoof trailers that introduce us to the main characters. Tugg Speedman (Ben Stiller) is an action star whose career has hit a downslide thanks to too many repetitive sequels. Jeff Portnoy (Jack Black) is a drug-addled comedian who makes lowbrow comedies in which he plays every member of a flatulent overweight family. Kirk Lazarus (Robert Downey, Jr.) is a "serious" actor who only does artsy projects and owns five Oscars to prove it.

The three - along with rapper Alpa Chino (Brandon T. Jackson) and neophyte Kevin Sandusky (Jay Baruchel) - come together for the filming of what is intended to be a very heavy war drama. Their British director, Damien Cockburn (Steve Coogan), doesn't have a lot of experience, and he has trouble reining in the egos of his actors. Profane studio head Les Grossman (Tom Cruise, in an overweight bodysuit so hairy that even Robin Williams would be aghast) ridicules Cockburn and demands that he take control of the project. Under the suggestion of Four Leaf Tayback (Nick Nolte), the veteran on whose experiences the fake film is based, Cockburn drops the actors into the jungles of Southeast Asia, where they come face-to-face with a real heroin operation, carried about by real bad guys who carry real semi-automatic weapons. He's filming things with hidden cameras, but that plan gets messed up quickly, leaving the cast on their own. Of the actors, only Speedman never quite figures out that the situation is real and no longer a set-up.

Tropic Thunder satirizes actor-ly concepts like "getting into character" and "being in the moment." With no actual knowledge of combat, the men are forced to essentially become their characters and use movie logic to survive, often with hilarious results. Even while being fired upon, they bicker about finding their motivation. It's a clever comedic idea: actors are so wrapped up in their own process that they essentially become oblivious to everything else. The screenplay takes it to ridiculous extremes, which is what makes it so effective. We've all heard stars talking to Mary Hart or Larry King about their process. That's a nebulous idea, I think, for non-actors, and the film suggests there's a touch of B.S. in there somewhere.

The other really great idea is to make fun of the notorious actor ego. Lazarus prides himself so much on being able to play anybody that he even dyes his skin in order to portray an African-American. When he realizes that the jeopardy is real, he still refuses to break character, much to the chagrin of Chino, his genuine African-American co-star. The best example of the film's satire of ego is also the most controversial: Speedman has made a blatant (and unsuccessful) grab for an Academy Award by playing a mentally challenged young man in a picture titled Simple Jack, which we see clips of. This joke has gotten Tropic Thunder in some hot water with the Special Olympics, who have formally protested the movie. They're missing the point; the joke is on actors, who often look to play mentally challenged characters because they know it leads to awards. In the case of Speedman, his attempt comes off more like Juliette Lewis in The Other Sister than like Tom Hanks in Forest Gump. The biggest laugh in the whole picture occurs when Lazarus chides Speedman for overdoing it, pointing out that the actors who get awards hold back a little bit. The scene ends with a punch line that is hysterical because it's so, so true. (It singles out a well-known actor.)

Toward the end, Tropic Thunder very slyly becomes the exact thing it's parodying. The actors find themselves genuinely living out all the clichés of war movies as they attempt to return to safety. There are dynamic battle scenes and rescue missions, all intentionally undercut by the fact that our heroes are crossing a line between performance and real life. You'd think that, after an hour and forty-five minutes of in-jokes and pop culture mimicry, the plot would peter out, but it doesn't. Stiller (who directed and co-wrote with Justin Theroux and Etan Cohen) is building toward something. There is an almost anti-Method attitude to the story, with the suggestion that perhaps these actors are more authentic when they're leaving their techniques behind. Stiller, Downey, and Black are all superb in conveying both the before and the after of their characters.

The time is right for a movie like this. More now than ever, the public is peeking behind the curtain of entertainment and understanding the machinations of it like never before. In addition to what I've already mentioned, the film also parodies studio politics, overly aggressive agents (Matthew McConaughey does his best work in years playing Speedman's), and the idea that stars are somehow legitimized by the number of awards they win. There have been quite a few show biz satires in the last few years, but this one is different. It pierces the armor of people who pretend to be someone else for a living, as well as the kind of weird combination insecurity and ego that undoubtedly drives them.

Of course, the more you know about actors and the movie industry, the funnier this is. Some jokes are obvious, others more subtle. Most of them are priceless. The year is only a little more than halfway over, but here is the funniest movie of the first eight months. If you read Entertainment Weekly from cover to cover, devour the bonus features on your DVDs, and watch the E! network religiously, then Tropic Thunder is a comedy custom made for you.

( 1/2 out of four)

Tropic Thunder is rated R for pervasive language including sexual references, violent content and drug material. The running time is 1 hour and 46 minutes.