At birth, Casilda, the 21 year old daughter of the Duke and Duchess of Plaza-Toro, was married by proxy to the then infant son and heir of the exceedingly wealthy King of Barataria. Unfortunately, the King became a rather bigoted Wesleyan Methodist, and the Grand Inquisitor, determined that such a trend should not continue, arranged for the heir to be kidnapped and taken to Venice. There he was placed in the care of a respectable Gondolier who raised him alongside his own son. Now Gondoliers themselves, his 'sons', Marco and Giuseppe, have recently selected a bride each - Gianetta and Tessa - from their large female following and are, therefore, upset to learn that, as heir apparent to the throne, one of them may have to sacrifice his bride in order to marry Casilda. She is not happy with the situation either as she is in love with Luiz. As Marco and Giuseppe's 'father' has since died, the only person who can identify the Prince is Inez who was his childhood nurse. When called upon to make her judgement she reveals that neither is in fact the King's son because when the baby was kidnapped she substituted her own child in his place. Therefore, the true heir to the throne of Barataria is Luiz. This proves very satisfactory to all parties as the Gondoliers can keep their brides and Casilda can both fulfil the proxy and marry the one she loves.