Composer Peter Bretter (Jason Segel) is in a five-year relationship with actress Sarah Marshall (Kristen Bell), who stars in a CSI-like television show titled Crime Scene: Scene of the Crime. One day, as Peter stands stark naked in their apartment, Sarah announces that she is breaking up with him. Devastated, and unable to banish his grief through one-night stands, Peter takes a trip to Hawaii. However, the vacation is ruined when he learns that Sarah and her new rock star boyfriend Aldous Snow (Russell Brand) are also guests of the resort. Taking pity on him, hotel concierge Rachel (Mila Kunis) offers him an expensive suite for free in exchange for cleaning up the room himself.
Peter begins spending time with Rachel and starts to develop feelings for her. Meanwhile, the relationship between Sarah and Aldous begins to falter. Much of the discord is triggered by the news that Sarah's TV show has been canceled and that Aldous is about to embark on a world tour with his group, Infant Sorrow, for 18 months. During a day of surf and sand Aldous and Peter run into each other and begin talking. Inadvertently, Aldous informs Peter that he and Sarah began having sex a full year before she broke up with Peter. When Peter confronts Sarah, she tells him she began feeling disconnected emotionally from him and she couldn't do anything to make their relationship work. Further exacerbating the situation is Sarah's obvious jealousy of the budding relationship between Peter and Rachel, while Peter (through observing Sarah's relationship with Aldous) begins to realize that his relationship with her wasn't as great as he remembered.
Sarah, Aldous, Peter, and Rachel later share an awkward dinner together. After the dinner, Peter takes Rachel back to his hotel room and they begin to have sex. Sarah hears them through the wall and initiates sex with Aldous, moaning loudly for the benefit of the couple next door, and Rachel and Peter turn the situation into a competition and become even louder. When Aldous realizes Sarah is clearly putting on a performance to provoke a reaction from Peter, he stops the sex and tells her the trip was a mistake as she's clearly not over Peter. They bicker furiously, prompting Aldous to announce that the relationship is over. The next day Peter encounters Aldous, and learns that he and Sarah have broken up and that he is flying back to England. Peter goes to Sarah's room to console her where she admits she still loves him and tries to rekindle their romance. The two start to engage in sexual activity, but Peter abruptly cuts it off because of his feelings for Rachel, and his now ambivalent feelings towards Sarah. Peter immediately goes to Rachel to confess what happened, but she is hurt and demands that he leave and never contact her again. Before leaving, Peter takes down a naked picture of Rachel from a local bar despite enduring a beat-down from the owner.
He flies back to Los Angeles and after a period of sadness and self-loathing, he begins working on his Dracula puppet comedy-rock opera, A Taste for Love. He sends an invitation to Rachel for the opening night performance. Although extremely hesitant at first, Rachel eventually decides to attend. After the performance Rachel congratulates Peter and tells him she's looking into attending school in the area. She leaves so Peter can bask in the success of his show, but quickly returns to Peter's dressing room to tell him she misses him. Peter tells her that he has also missed her too. The film ends as they embrace and kiss.
Who demands that Peter never contact her again?
Rachel