Among the passengers entering the island's general store/hotel run by Joe Horn and his native wife are Doctor Robert MacPhail, a philosopher, and his wife; Alfred and Martha Davidson, a missionary couple, among others.

Entertaining the Marines in her state room is Miss Sadie Thompson, a prostitute, who enjoys the company of men, playing loud floozy music and cigarette smoking. She quickly catches the eye of Sergeant Tim O'Hara , who becomes interested in her, but the disapproval of Davidson, who objects to her immoral ways.

At first Davidson forces himself upon her to reform, but all he finds is that his religious persistence annoys her and that Sadie can be equally demanding and powerful as he.

Sadie tries to meet him half way when she learns that she must return to San Francisco and serve a three year prison term, and becomes bitter when Davidson won't give in to her pleas.