As its advertisements promise, “Forgetting Sarah Marshall” is “from the guys who brought you ‘The 40 Year Old Virgin’ and ‘Knocked Up.’” This newest film is the heir to the Judd Apatow comedy throne; it has some big shoes to fill.

“Forgetting Sarah Marshall” — directed by Nicholas Stoller — stars Jason Segel, also the film’s writer, as broken-hearted Peter Bretter. He wallows in his apartment after breaking up with his television-star girlfriend, Sarah Marshall (Kristen Bell). His sorrow turns pathetically into depression after he fails to get over her through a series of one-night-stands. So Peter heads to Hawaii to recover on the advice of his brother, Brian (Bill Hader).

Sarah has the same idea. The two accidentally meet at the Hawaiian resort moments after Peter arrives. To add to the awkwardness, Sarah has her new boyfriend in tow: lothario rocker Aldous Snow (Russell Brand).

Soon after, Peter meets Rachel (Mila Kunis), a cute front-desk clerk and perfect foil to Sarah’s wannabe-movie-star attitude. Rachel, fresh off of a breakup of her own, sympathizes with Peter, helping him to relax and move forward. Segel and Kunis have on-screen chemistry from the start, and their characters’ relationship is believable.

Rounding out the rest of the cast are a few Apatow film staples. Paul Rudd plays a stoner surfing instructor, while Hader attempts, to no avail, to give Peter advice through cell phone and video chat. “Superbad” star Jonah Hill also turns up as an obsessive waiter, sucking up to Aldous whenever he gets the opportunity.

The funniest cast member, however, is virgin newlywed Darald (Jack McBrayer), who’s left agape at his bride’s sexual desires until he seeks advice from Aldous — one of the film’s funniest moments.

Overall, the main cast members fit into their roles effectively. Bell and Kunis — best known for their television roles on “Veronica Mars” and “That 70s Show,” respectively — play off of each other well as the two begin competing for Peter’s attention. Bell nails Sarah as a smooth blend of self-superiority and vulnerability.

Brand hilariously portrays a self-absorbed British rocker whose carefree attitude leads to further laughs. In one scene, a waiter asks if Aldous has listened to his demo. Aldous responds, “I was going to listen to that, but then I just carried on living my life.”

The movie, as with other Apatow flicks, seamlessly merges raunchy comedy with real, romantic emotions. An impromptu contest to determine who can have the loudest sex is followed by a tender, relationship-affirming moment. This duality makes the movie a success.

Still, “Forgetting Sarah Marshall” has its faults. The relationship between Aldous and Sarah is never believable and seems forced at times. It also takes time to settle into its rhythm. Some viewers may find it difficult to invest in the relationship between Peter and Sarah — which fizzles before the audience has much knowledge of their history together.

Segel’s writing alone makes the movie worthwhile. While the plot may seem mundane, the snappy dialogue and character development more than makes up for it. The plot’s quirks are some of the best moments of the film, such as Peter penning a Dracula musical, complete with a cast of puppets. Segel doesn’t shy away from anything, choosing to have his own character appear frontally nude twice.

“Forgetting Sarah Marshall” is no “Knocked Up,” but it doesn’t pretend to be — it’s entertaining the way it is.

“Forgetting Sarah Marshall” was written by Jason Segel and directed by Nicholas Stoller. It received three and a half out of four stars.