On the eve of the elections, a candidate for mayor of a small English town turns to a familiar private detective for help exposing a blackmailer. Sam Quarrey takes the job without much enthusiasm—especially since in pursuit of the criminal he has to go on a cruise, sharing a cabin with a stranger: a very strange woman.
What will the stubborn passenger Sam is forced to pass off as his wife turn out to be—an ally or an enemy? And what secrets will the investigation aboard the steamboat uncover?