The first twenty minutes of Tropic Thunder represent some of the funniest to grace American silver-screens in the last five years. For this short amount of time, the movie does exactly what a great modern comedy should: provide non-stop laughs, smart references, and a relatable context. Introducing us to its focal three characters—actors Tugg Speedman (Ben Stiller), Jeff Portnoy (Jack Black), and Kirk Lazarus (Robert Downey Jr.)—Tropic Thunder nails today’s film-industry on the nose. The trio is a hot item in Hollywood: Speedman is a popular action-star who occasionally sees a flair for the dramatic (he makes a Patch Adams-like turn when he goofily plays a mentally-challenged farmer in the movie Simple Jack), Portnoy is a closet heroin-addict whose comedies are hugely successful, and Lazarus is a committed (so much so that he decides to become medically African-American for his latest role) method-actor who stars in controversial indie projects. All together, the characters encompass everything that is ridiculous about big-budget American moviemaking and, at the same time, everything that is glorious about the art: they’re all a bit loopy, but they’re kinda brilliant from a marketing-perspective.

     The opening scenes of Tropic Thunder work because they merely dwell on the carefully-observed, nuance-filled traits of the characters. The movie opens to a dynamite showcase of this process: three trailers that provide examples of each of the actors’ work. I would not dare reveal the contents of these or the titles of the movies they represent, but will say that they feel almost eerily authentic in the ways that they rip on recent Hollywood projects. Arnold Schwarzenegger and Eddie Murphy make for amusing, if obvious targets in Speedman and Portnoy’s clips. The Lazarus trailer is the best of the bunch, however, taking more than a few jabs at Brokeback Mountain and featuring a cameo from a certain metrosexual superhero-playing star.

     The laughs and pointed-observations don’t stop coming when the trailers end. The audience’s attention is quickly directed to a humorously-staged, big-budget, Apocalypse Now-style Vietnam War film that the actors are shooting in the remote jungles of Southeast Asia. The project is running out of money quickly and isn’t looking up in terms of quality, either, as evidenced by the director’s (Steve Coogan) eagerness to get his cast to settle for mediocre takes. This involves a particularly hilarious bit in which Speedman is shot repeatedly, somehow surviving as blood flees his system, only to share an emotional moment with Lazarus’ double-talking black soldier.

     The studio and the filmmakers soon reach a point of financial and creative gridlock, however, and the project looks like it may go under. To save the movie, the cast and crew follow the advice of ex-POW screenwriter Four Leaf Tayback (Nick Nolte) by deciding to shoot the entire thing “guerilla-style” on handheld cameras with only the real pyrotechnics they have been afforded. What they don’t realize, however, is that there are real resistance-fighters/drug-lords occupying the jungles where they decide to film and, because they are dressed as American soldiers and carry fake weapons, they will instantly become targets for actual warfare. (Only complicating matters further is the fact that the group assumes its newfound opponents to be studio-plants designed enhance the authenticity of its ensuing theatrics.)

     Despite the inspired plot-making displayed by the movie-within-a-movie-within-a-movie structure developed, Tropic Thunder becomes wildly hit-and-miss as soon as the actors and the drug-lords become involved in full-scale battle. Instead of continuing to focus on the personalities of its characters, the film largely morphs into the very inept action picture it thinks it’s spoofing. Yes, there is some inspired acting on display (particularly on the part of Downey Jr.), but the script becomes largely unfunny. Comic details are ignored in favor of loud action sequences, the most inept of which being the movie’s finale, in which the unscathed portion of the movie’s cast must rescue that which has been imprisoned by the drug-lords. Tropic Thunder also reduces itself to implementing crude humor at moments in order to fill space, too, with plenty of lewd instances like one in which Portnoy frantically describes a sexual act he would perform in order to score much-needed heroin that the isolated jungle has deprived him of.

     Yes, Tropic Thunder’s final two acts are not without their merits, but these come few and far between. Most are tied to three reoccurring situations: the drug-lords’ unexpected glee when they discover Speedman to be their POW given that the otherwise-critically-lambasted Simple Jack is the only VHS-tape they own, the crazed antics of studio-exec Len Grossman (a wonderfully self-deprecating Tom Cruise), and the desperately misunderstanding attempts of agent Rick Peck (Matthew McConaughey) to remain employed for Tugg. For at least the last hour of Tropic Thunder’s 107 minutes (if not the last hour and a half), the movie is nowhere near as consistent as it is during its opening passages. As bullets fly, drug-fields explode, and action-movie clichés are explored in overwhelming abundance, viewers will find themselves longing for the greatness that the film was able to capture for a short period of time. Yes, Tropic Thunder is an amusing effort that ultimately proves worth seeing, but it’s a somewhat disquieting exercise to think about how much better it could’ve been.