The Rum Diary is about the misadventures a writer experiences in Puerto Rico and finally finding who he is. Paul Kemp(Johnny Depp)is a writer trying to get someone to notice his book with no luck. He finds himself in Puerto Rico writing articles for a newspaper. While working for the paper, he is staying a hotel. While there, he runs into Chenault(Amber Heard)who is engaged to Sanderson(Aaron Eckhart). Sanderson recruits Kemp to write articles for his real estate venture. Along his journey, Kemp meets Sala(Michael Rispoli), Lotterman(Richard Jenkins), and Moberg(Giovanni Ribisi). Lotterman is his editor for the newspaper. Moberg is another reporter for the paper, and Sala is someone Kemp meets and befriends at the hotel.
Kemp wants to write about the real things that are happening in Puerto Rico, but Lotterman refuses his articles and tells him to continue to write what he told him to. Kemp and Sala gets in trouble with the locals and end up in jail. Sanderson bails them out under conditions that they will continue to work on the real estate scheme that him and his crew have going on.
Sanderson is upset that Kemp involved Sala in the scheme and tells him that his part in the scheme is over with. Sanderson also retracts the bail money so Kemp and Sala are wanted men. Chenault leaves Sanderson. They find out that the paper has gone out of business. Having nowhere to turn Kemp and Sala do some drugs. While in a drug trance, Kemp realizes he can write about the ongoings of Sanderson and Lotterman.
Kemp and Sala decide to bet on the cock fights to have the money to print his articles. They win, but find out the printers have been taken. Finally giving up in Puerto Rico, Kemp heads back to the states, marries Chenault and finally finds his success as an author.