Tropic Thunder is a hilarious sendup of war moves, particularly Apocalypse Now (not to mention Hearts of Darkness.) It is also a sendup of Hollywood itself, a place where people are so busy putting on an act that they lose all connection with reality.
The story focuses on four actors: a washed-up action hero (Ben Stiller), a fat comedian who specializes in fart jokes (Jack Black), a rapper best known for promoting a beverage called “Booty Sweat” (Brandon T. Jackson) and an obsessive method actor (Robert Downey Jr.) who dyes his skin to play an African-American and insists on staying in character even off camera. They are the leads in a doomed movie based on the memoirs of a Vietnam War hero (Nick Nolte).
Filming is going nowhere due to the titanic egos and general incompetence of the actors, so the director decides to dump them somewhere deep in the jungle and film them with hidden cameras, hoping for some documentary-style realism. Things go horribly wrong and soon the actors are being hunted by drug smugglers, all the while assuming that this is just part of the movie.
There is some really funny stuff here, but you should be aware that some of the humor is tasteless or just plain gross.
Though some of the jokes are deliberately politically incorrect, one reaction to the movie probably surprised the writers. Regrettably, a number of advocates for the mentally disabled are making asses of themselves by picketing the movie, claiming that it portrays mentally disabled people in a negative light.
Actually there are no mentally disabled characters in the movie. There is only a very short sequence of an actor hoping to win an Oscar by playing a retarded man, and doing a very bad job of it. Obviously the point here is not to make fun of the disabled, but to make fun of the Hollywood notion that there is some special merit in playing them.