Frank McCloud is an embittered war veteran who travels to Key Largo in Florida in order to meet Nora Temple, the wife of a friend of his who died in the war. He plans to stay at a hotel run by relatives of Nora's, but discovers that it has been taken over by vicious gengster Johnny Rocco. McCloud doesn't want to get involved in opposing Rocco, having lost any idealism in the war, but events take a more serious turn and he is forced to rethink his beliefs.