The beautiful princess, Marie de la Bonafain, is being forced by her uncle to marry a Spanish gentleman, Don Carlos de Braganza. Unhappy with the arrangements, she refuses to see Don Carlos when he comes to visit. Marie's uncle is furious and demands that she marry in the next week, threatening her with prison for disobedience. Alone in her room, Marie tries to think of an escape. She decides to take the identity of her maid, Marietta, and sail to New Orleans as a Casquette Girl (a sort of mail-order bride), to marry a farmer there.
All goes well, and Marie (now known as Marietta) sails away to the strains of Prayer. Furious at the discovery of his niece's departure, her Uncle and Don Carlos board a ship and set out to overtake her.
A short distance from Louisiana, Marietta's ship is attacked and taken by pirates. As the pirates decide how to divide the girls between them, Captain Warrington and his band of mercenaries come in search of the villains, singing Tramp, Tramp, Tramp. The pirates scatter, and a fast battle ensues, with the mercenaries victorious.
Once the men and girls are camped for the night, Captain Warrington entertains them all by singing Owl and the Pole Cat. His cocky attitude annoys Marietta, even though she is attracted to him from the first. Warrington, however, informs her that he has no interest in a "petticoat" hanging around him and enjoys his life as a sodier too much to be bothered with women. He soon realizes, though, that she is not what she pretends to be and tries to find her out. She evades all of his questions, however, so he sings 'Neath the Southern Moon and retires.
Upon their arrival in New Orleans, the girls are taken to the church, where all of the men are gathered, waiting to choose a wife. Men flock to Marietta, but she refuses them all, telling them that she hasn't the slightest intention of marrying. This goes against the rules of her contract as a Casquette Girl, so the governor commands Captain Warrington to escort her to a boarding house, where she will reside until they can send her back to France. Warrington pays a band of gypsies to sing Italian Street Song for him, all the while bragging about his wonderful voice. Fed up with his bragging, she bursts into song with the gypsies and shocks him into silence.
The next day, Marietta finds a job singing in a marionette theater. The Captain finds her there and, after agreeing to forget their past differences, they lunch together. As they eat, a rider comes up from the docks with news. A ship has landed from France, with a reward offer for the return of the girl calling herself Marietta. Warrington sends Marietta to his boat around the corner and sends the crowd to the theater.
In an effort to hide her, Marietta and the Captain sail down the river to his band's headquarters. He sings I'm Falling in Love with Someone to her and asks her to sing it back to him, but she promises, instead, to sing her own love song to him the next day. As they pull up to the dock, soldiers take Marietta away to the Governor's palace, informing Warrington that Marietta is a princess in disguise.
That night, a ball is given in the honor of her uncle (a Prince), Princess Marie and Don Carlos. Marie's uncle informs her that they will be sailing that night for France and that Warrington will be branded a traitor and shot if he tries to see her. She tries to send word to him to stay away from the ball, but he has already arrived. They dance together, and she tells him that she is not leaving for three days, promising to meet him the next evening to sing her love song to him.
After the dance is finished, Marie walks sadly towards the stairs, in one of the best Mac & Eddy moments ever. Suddenly, she asks the musicians to play her love song. She knows that if she does not sing it to Warrington now, then there will never be another chance. She sings Ah, Sweet Mystery of Life, crying as she walks up the stairs. Warrington sings it back to her, and then they sing it together.
Back in her room, Marie cries by herself, only to look up and see Warrington come in through the window. They escape through the garden and ride into the West, to start a new life together.