Ewart Gray (Donald Sinden) is the new Resident-in-Charge of the Welcome Islands in the Indian Ocean. The Islands are full of life, but the only other Europeans are the "sanctimonious, psalm-singing" brother and sister missionary team of Martha Jones (Glynis Johns) and Reverend Owen Jones (Paul Rogers), and Edward "Honorable Ted" Wilson (Robert Newton) - a hard-drinking, womanizing social outcast whose English family pays him to stay away. Martha and Ted become an unlikely team when cholera threatens the islands and they must do their best to stop its spread.