At one point in the raucous new comedy “Get Him to the Greek,” Russell Brand’s rock star character, Aldous Snow, has whipped the “Today Show” crowd into such a frenzy that grown adults are gleefully singing, “I’ve got the clap!” in the street and on live television.

Such is the life of a rock star. They possess incredible power that often does little to advance collective humanity and a lot to satiate their own egos, addictions and libidos. Reprising his knockout cameo in 2008’s “Forgetting Sarah Marshall,” Mr. Brand has created in Aldous Snow a near-perfect embodiment of the narcissistic excesses of rock and roll, certainly the closest since the fellows of Spinal Tap cranked their legendary amps up to 11.

The yin to the degenerate rocker’s yang is Aaron Green, a soft and pudgy record company flack played by Jonah Hill, the most well-rounded – both physically and in terms of acting chops – of Judd Apatow’s (one of the film’s producers) legion of geeks. Here, Mr. Hill manages to be dorky enough for the comedy to work and hip enough to hang with the cool kids.

Aaron is charged with the task of traveling to London to retrieve the mercurial rock star, whose career and personal life have reached their nadirs. This is Aaron’s big chance to prove himself to his boss, Sergio (played by Sean “P. Diddy” Combs with reckless self-mockery). The two have 72 hours to make a stop in New York for the “Today Show” appearance and then carry on to Los Angeles for an anniversary concert commemorating Aldous’ legendary performance at the Greek Theater ten years before. The two get there with little time to spare, of course, and not before a few unplanned detours, forced learning experiences and drug-fueled hijinks.

Writer-director Nicholas Stoller, who also helmed “Forgetting Sarah Marshall,” attacks this project with all the subtlety of a rock anthem – an appropriate touch for the subject matter.  The raunchy approach works well for the first two-thirds of the film, when it’s a joy to see what Brand will improvise next (he tells Aaron, “Your mind is filled with lollipops, rainbows, cheese and wonder”); how each song’s
lyrics will outdo the last (there is a heartfelt, sing-along ballad about his “Banger, Beans and Mash”); or what vomit-strewn, awkward mess the lovable chubby kid will get into next (thankfully, without cheap laughs owing directly to his portliness).

Even the best comedies, however, often struggle at the finish line, and “Get Him to the Greek” is no exception. Throughout all the fun, Mr. Stoller struggles to identify which of his two main characters is the protagonist. After learning to love the bad-boy rocker, it feels disingenuous when Aldous inevitably snaps out of his narcissism to grasp the important life-lesson. And the story finally goes off the rails with a strange scene in which Aldous tries to fix Aaron’s relationship with his live-in girlfriend (Elisabeth Moss from “Mad Men”) by, let us just say, involving all three of them in the reconciliation.

Fortunately, by then “Get Him to the Greek” has produced enough laughter to forgive the newfound sentimentality, however twisted it may be.

1hr 49min. Rated R for considerable language, drug use and sexual content.