There have been countless action-adventure films that were unintentionally hilarious (thank you Steven Seagal). Few, however, have done it purposely as well as "Tropic Thunder."

Star-studded, explosion-filled and decidedly non-PC, Ben Stiller's mockery of Hollywood movie-making may be his best work since "There's Something About Mary." The jokes don't always hit their mark -- not by a longshot -- but within moments there's usually another one that will make you forget about the dud.

The movie gets off to a clever start with some faux movie previews (and a Tobey Maguire cameo) and then dumps us into a Vietnamese jungle, where Stiller's once-popular action star Tugg Speedman is trying to jump start his career with a war epic based on the story of Nick Nolte's heroic Four Leaf Tayback.

The cast is headlined by five-time Oscar winner Kirk Lazarus (Robert Downey Jr.), a method actor with a tendency to lose himself in every role. In this one, he undergoes skin pigmentation surgery so he can play an African-American soldier.

There's also comedian Jeff Portnoy (Jack Black), hoping to elevate his career past fart jokes, and a rap star turned actor (Brandon T. Jackson), who is mostly annoyed to hear the fake ghetto banter coming from Lazarus. Jay Baruchel ("Knocked Up") rounds out the cast as the token non-prima donna.

When filming doesn't quite go as planned, the movie's over-his-head director opts to drop his cast in the middle of a real jungle and film them with hidden cameras. A nearby drug operation, however, takes offense and the Hollywood stars find themselves in a real battle -- which they think is still merely special effects.

The most fun in "Tropic Thunder" comes from watching Downey Jr., who somehow manages to play his part to the hilt without becoming a parody. It's been a big summer for Downey Jr., who has succeeded in two roles (superhero and black man) most people never expected him to play.

The next best thing is a bombastic cameo by Tom Cruise (in a fat suit), who hits all the right notes as profane producer Les Grossman.

Though "Tropic Thunder" has faced protests for insensitive portrayals of the mentally handicapped, the film is effectively a send-up of the movie industry itself. Big laughs and big explosions make it a fitting end to the summer movie season.

Rental Recommendation: There aren't quite as many laughs, but "Apocalypse Now" is another Vietnam movie with a famously disastrous filming. Grade: A