Peter (Jason Segal), a successful composer is heart-broken and devastated when his girlfriend of five years, Sarah Marshall (Kristen Bell), the famous star of a popular crime show, splits up with him. Peter is even more devastated at the fact that she has left him to be with renowned rock star Aldous Snow (Russell Brand).

Peter soon goes into turmoil and becomes a recluse until his step brother convinces him to take a much needed vacation. Peter agrees and goes to Hawaii, but when checking in at his hotel he comes across Sarah and her new lover Aldous. But Peter refuses to leave and so does Sarah.

Will Peter be able to forget about Sarah Marshall despite being on the same island as her and has he possibly found a new love in hotel receptionist Rachel (Mila Kunis)?