Forgetting Sarah Marshall is a romantic comedy. It's about a man named Peter who is a music composer for a television show that his girlfriend Sarah plays on. The opening scene shows Sarah coming home to find Peter naked. She tells Peter their 5-year relationship is over. Peter is devestated. Peter falls into a slump where he can't compose. While at work he breaks a movie screen. He is fired. He sits at home and cries over his loss. He thinks of all the fun times he had with Sarah. His step brother, Brian, tells him he needs to get back on the horse and start dating again. After several one-night stands, he decides he needs to get away.
Peter heads to Hawaii. When he checks into the hotel, he meets the receptionist, Rachel Jansen. She is a very sweet girl. As she is telling him there are no rooms available, Sarah Marshall walks by with her new boyfriend, Aldous Snow. He explains that she was his ex, and now she is happy with Aldous, and he is alone. Rachel tells Peter that a suite is available for him. She saved him from humiliation in front of Sarah.
After running into Sarah and Aldous many times, Peter finds companionship in Rachel. She is fun, attractive and has helped him keep his mind off of Sarah. She shows him around the island and all that it has to offer. There is a connection between them.
Things start to get rocky with Sarah and Aldous after she finds out he has cheated on her numerous times and doesn't plan to stop. Sarah goes to find comfort in Peter. She starts trying to engage in sexual activity, but he stops her advances. Right afterward Peter went to tell Rachel what happened. She is furious and breaks up with him.
Peter flies back to the states and puts on his play about Vampires. He invites Rachel, and she shows up. They start to rekindle what they had in Hawaii.