A steamship originating from San Francisco has just docked on the South Pacific island of Tutuila at the port of Pago Pago, where some US Marines are stationed. Among the passengers is Sadie Thompson, for who Pago Pago is only a transit stop in that she is on her way to Apia on the nearby island of Upolu, where she is to start work for the steamship company itself. Circumstances force her to stay in Pago Pago additional days, which places her in a difficult situation as she starts to run out of money. Sadie is either fun-loving or a harlot depending on one's perspective; Mr. and Mrs. Alfred Davidson, fellow passengers on that steamship, hold the latter perspective. Davidson, a self-professed deacon, is a reformer who has the ear of all the governors representing the South Pacific islands. His self-righteous and sanctimonious beliefs and attitudes include sinners needing to show true penance to achieve ultimate salvation. He believes that Sadie is a prostitute who will continue to ply her trade in Apia. He is only half right as she was a prostitute in San Francisco, until she left to start life anew. Davidson confronts her with his beliefs about her, threatening to get the governor to deport her back to San Francisco, which would be her worst nightmare as she is also escaping other things in San Francisco. By this time, Sadie and U.S. Marine Sergeant Tim O'Hara have fallen in love; Tim knows her past but still wants to marry her and take her to start a new life in Sydney. Tim does what he can to stop the powerful Davidson, who is not as righteous as he wants the world to believe.