Peter is a composer and a likable sad sack who's devastated when his girlfriend of five years, Sarah Marshall, the star of a cheesy CSI-style crime show, dumps him. He weeps, he rails, he mopes. Finally, his step-brother Brian suggests Hawaii, so Peter heads for a resort on Oahu where, as he's checking in, he sees Sarah and her new beau, Aldous, a polymorphously perverse English rocker. The weeping and moping start again, until Peter is rescued by Rachel, a thoughtful hotel clerk who invites him to a luau and to hang out. Although he constantly runs into Sarah and Aldous, Peter starts to come alive again. Will Sarah realize what she's lost, and what about Rachel?

When Sarah Marshall dumps aspiring musician Peter Bretter for rock star Aldous Snow, Peter's world comes crashing down. His best friend suggests that Peter should get away from everything and to fly off to Hawaii to escape all his problems. After arriving in Hawaii and meeting the beautiful Rachel Jansen, Peter is shocked to see not only Aldous Snow in Hawaii, but also Sarah Marshall.