New Moon starts out as the Aristocratic passengers of a ship sailing to Louisiana are having a party.  However when Marianne (MacDonald) is asked to sing, her song gets interrupted by the noise of the less then aristocratic passengers below deck, who were put there to be bon servants due to crimes.  Despite his attempts to convince his passengers that the noise is from cattle, Marianne is not fooled, especially when it appears the "cattle" are singing their own version of the song she had been trying to sing.

Their leader is Charles Mission, a Duke disguised as his butler who was sentenced to being a bon servant for singing revolutionary "ditties" in the streets of France.  After discussing the matter with some sailors sent to discipline the men below they reluctantly agree to see if he could have an audience with the captain.  After the sailors leave Mission is cornered by two other men who have come to realize who he really is.  He urges them to continue to keep it a secret and tells them the plan he has for getting away from Louisiana.

While awaiting the Captain, he finds himself meeting Marianne who has come to complain to the captain for her Aunt who has had trouble sleeping due to the noise the men below were making.  He quickly grabs an officer's coat, since despite his disguise his manners when greeting a lady do not include being without a coat!  This however gives Marianne the impression that he is indeed an officer.

Whenever they reach land, Marianne searches for the officer she met in the Captain's office and is disappointed when she doesn't spot him before she is greeted by the Governor who knew her when she was younger.

She returns to the plantation that her father owned in Louisiana and awakes the next morning only to find that the Officer she met was actually a bon servant working in her house.  She is angry but decides to keep him in her service.

Soon it is Mardi Gra and she holds a large party, gaining many compliments.  When she compliments her lead servant on the festivities he tells her that it was Mission and not he who did most of the planning.  Angry, she confronts Mission only to find out that he planned it according to the plans her mother set out for Marianne's birthday when the girl was very small, up to and including the song she sang.

As Marianne says good-bye to her guests, one of the ladies compliments her on her servant and tries to buy him, which gives the women all kinds of assumptions.  At the end the women relents with the comment that should she change her mind, to send him on her fastest horse.

Marianne leaves the party and retreats into her garden and begins to watch the slaves as they hold their own celebration.  Mission follows her, with the intention of returning the shaw she dropped near the house but begins to explain to her what the slaves are doing.  The pair end up singing along with the slaves and the song causes an emotional embrace and kiss, after which Marianne rushes away.

When she arrived she is greeted by the Governor and an Investigator from Paris who has been trailing the Duke de Vidiers who is thought to be a traitor to the crown of France. Marianne hears his story, and it dawns on her that Charles Mission must be the Duke himself.  Although she dislikes the character that has been presented to her by the Investigator's reports as well as her Aunt's gossip, she can't help but want to protect him.  She tells the Investigator that she does not know where he is, and after the man leaves, she quickly writes a letter to the quest who wished to purchase Mission earlier and calls him in.  After giving him the news she turns her back on him, so he doesn't see her true emotions.  He leaves her, angry and disappointed.

However, on his way to go he meets up with Alexander, one of the men who had known his true identity earlier.  Alexander has been watching the docks and the New Moon, a ship that Mission had planned on using to start his own revolt against the crown.  He gathers some "stouthearted men" and take the ship.  However, He finds out about the Investigator and now believes that Marianne was trying to protect him, not sending him away because of the kiss.

Meanwhile, Marianne and her Aunt have decided to return to France.  Her Aunt is happy the Duke got away (not knowing it was Mission) and the Investigator is less then happy and decides to return with them.  However, their ship is boarded by the men of the "New Moon" which had suffered damages and Marianne comes to the Captain's room hoping the buy the freedom of the crew only to be met with Mission.  They argue over what she is doing and she leaves.

A storm causes the boat severe damage and the crew has to try to make land at a near by uninhabited island.  While most of the men from the New Moon and the girls traveling to be brides were saved, several of the crew of the second ship did not make it.  The Investigator decides to make the most of this and joins in on the creation of a town.

However, it is not long before building is not enough to keep the men occupied and fights break out.  The Reverend in charge of the brides decides that the best course of action is to ask the men to marry and start families.  Colonize the land.  Mission agrees, and makes an announcement.  However this causes problems for both him and Marianne as the two attract several suitors. 

Marianne, tired of being "wooed" decides to take the matter up with Mission who is the community's leader.  However, he cannot do anything and offers up only one solution: Marry him.  She declines at first but soon agrees knowing she will get no break from the suitors if she remains unwed.

The two marry, although not happily and put up a face for their friends who think they are happy.  After retiring to their cottage for their wedding night, they go to their separate rooms after a short discussion.  Marianne finds out that the Duke she assumed was his true self was just her Aunt misinterpret his words.  This leaves her slightly offkilter as it changes how she looks at him.  After an accident involving the large door lock, she receives a small kiss but soon a sound rings out.  Mission rushes to join the men at the beach where a ship has been spotted.

Marianne waits at the cottage and he returns to tell her the good news: The revolutionaries have won!  A General comes to offer his congratulations to the couple, who happily now realize they love each other and the two celebrate that as everyone celebrates the dawning of a new French government.