Get Him To The Greek is a spin-off from Forgetting Sarah Marshall starring Russell Brand as Aldous Snow.
Here’s the Wiki plot:
Aldous Snow (Russell Brand) releases a song about problems in Africa called “African Child”. The song and music video are seen as horrible and called “the worst thing to happen to Africa since apartheid”. The single ruins Snow’s career, and as a result he starts drinking and doing drugs excessively. His girlfriend Jackie Q (Rose Byrne), with whom he had a son named Naples, also leaves him, taking custody of Naples.
Aaron Green (Jonah Hill) is a driven, idealistic young college graduate who works as an intern at Capitol Records. He lives with his girlfriend, a doctor, but they do not get to see each other often due to their different sleep schedules. Capitol Records has lost a lot of money, so the executive of the branch Sergio (Sean Combs) asks for ideas. Green proposes to have Snow play at the Greek Theater for the tenth anniversary of his performance. Sergio is impressed with the idea and charges Aaron with getting Aldous Snow to the theatre. Hilarity ensues as idealistic Aaron takes drug-addicted Aldous from London to Los Angeles.