Plot:
In 2009, British rock star Aldous Snow (Russell Brand) releases an album and a titular single—"African Child", which is a commercial and critical failure. Despite having been free of alcohol and other drugs for seven years, along with his pop-star girlfriend Jackie Q (Rose Byrne), when, in an interview, she drunkenly declares they have a boring life, he relapses—which effectively ends his relationship with her, makes him lose custody of their son, Naples, and sabotages his career. Meanwhile, in Los Angeles, Aaron Green (Jonah Hill) works as a talent scout at Pinnacle Records, a successful record company. He lives with his girlfriend, Daphne (Elisabeth Moss), a doctor. Pinnacle Records is performing badly as a result of poor record sales, and the head of the company, Sergio Roma (Sean Combs), asks for ideas. Green proposes that Aldous Snow play at the Greek Theatre in Los Angeles on the tenth anniversary of his legendary performance there in 1999.
Sergio sends Aaron to London to escort Aldous to the performance. Before he leaves, Daphne informs Aaron that she has received a job offer in Seattle and that they are moving there, which leads to an argument resulting in an apparent break-up. After retrieving Aldous from his apartment, Aaron learns that Aldous had not been expecting him and that he had thought the concert was not for another two months. They then bar-hop across the city and Aaron futilely tries to get Aldous to catch one of many missed flights. Daphne calls Aaron to apologize for the fight, only to learn that Aaron believes that they are broken up. Throughout his partying, Aaron's cellphone accidentally calls Daphne periodically, informing her of Aaron's activities. Aaron and Aldous travel to New York for Aldous's appearance on Today. To keep Aldous sober for his performance, Aaron imbibes all of Aldous's whiskey and marijuana. Minutes before the performance, Aldous realizes he is unable to remember the lyrics to his most recent and unpopular song, "African Child", and replaces it with an older hit, "The Clap", to cheers and excitement from the audience.
About to embark on a flight to Los Angeles, Aldous forces Aaron to smuggle heroin in his rectum. During their travels, Aaron learns that Aldous has become miserable and lonely, as he misses his son and has been alienated from his own father Jonathan (Colm Meaney) for years. Aaron suggests he visit him after the show; instead Aldous insists they go to Las Vegas to see Jonathan. Sergio soon arrives, with plans to "mindfuck" Aldous to Los Angeles. Sergio hooks up Aaron with a sexually violent girl named Destiny (Carla Gallo), who takes him to a hotel room and rapes him. After Aaron tells Aldous that he has been raped, Aldous gives him a "Jeffrey", a joint described as "a Neapolitan of drugs". Aaron panics and starts to have a bad trip, believing he is having a heart attack. Jonathan makes the trip worse by agreeing that Aaron is having a heart attack while Aldous attempts to calm Aaron down, primarily by the comforting sensation of stroking the furry walls of the hotel suite. Aldous fights with his father; Sergio (who is also high) jumps in the fight, and inadvertently sets the lounge on fire.
Aldous attempts to help Aaron by giving him an adrenaline shot and they run out of the hotel, chased by Sergio, who is hit by a car but comes out unharmed. Aldous and Aaron flee to Los Angeles, where Aldous visits Jackie Q. She has been sleeping with Metallica's drummer, Lars Ulrich (playing himself) and confesses that Naples is not actually his biological son, but instead is a photographer's son. This depresses him even further. Meanwhile, Aaron goes to his home to apologize to Daphne. They are interrupted when Aldous arrives at their house and proposes that he, Aaron, and Daphne engage in a threesome; Daphne (who is mad at Aaron) agrees and Aaron hesitantly goes along. During the threesome, Aaron angrily decides to kiss Aldous, breaking it up. Daphne and Aaron both immediately regret the threesome, and Aaron angrily tells Aldous to go, criticizing Aldous's overall mental state. Instead of preparing for his show, Aldous goes to the rooftop of the Standard Hotel in downtown Los Angeles, and calls Aaron, threatening to jump. Aaron rushes to the hotel, and attempts to coax Aldous down from the roof.
Instead, Aldous jumps into a rooftop pool several floors down, accidentally breaking an arm. Aldous tells Aaron that he is lonely, sad and embarrassed, but Aaron reminds Aldous that thousands of fans love him and are waiting just to see him. Aldous decides to perform at the Greek Theatre despite his injury, even though Aaron pleads for him to go to the hospital. Upon their arrival, Sergio offers Aaron drugs to give to Aldous so he will not cancel the concert. Aaron, tired of the abuse Sergio has given him, refuses and quits his job on the spot, much to Sergio's dismay. Aaron walks stage-side with Aldous, trying to convince him to go to the hospital instead. However, Aaron sees how happy Aldous is while performing and heads home to reconcile with Daphne. Months later in Seattle (where Aaron and Daphne have moved), Aldous, sober once again, has returned to fame with a single "Furry Walls" produced by Aaron (now his official producer) based on events from their night in Las Vegas, performing on the VH1 Storytellers program.