The light opera mixes spoken word and powerful operatic verses as the girl, Marie, accepts her life as the battlefield nurse and moral compass for a passionate troop of French soldiers. While performing her duties she is put in danger, but has her life saved by Tonio, a Swiss soldier.

Marie has found true love but is bound as a la fille du regiment to only marry a member of the second regiment. Tonio risks being branded a traitor and a spy and joins the second regiment.

News soon arrives that she is in fact related to the marchioness of the castle by which she was originally found. She follows her blood relatives leaving her beloved regiment and her role as la fille du regiment behind.

In the meantime, Tonio has been outed as a Swiss and faces execution. Once Marie hears the news she rushes to confront the regiment and tell how he saved her and reveal why he joined, because he loves her. Saved, Tonio is allowed to continue his service with the regiment, Marie returns to her place with the marchioness.

Life at the castle is not what Marie enjoys, despite her new found riches. Her only joy is talking with Sulpice, a sergeant from her old regiment. Her life has been set without her consent and she is to marry the son of the duchess of Craquitorpi.

Just before her wedding the second regiment shows up with Tonio as an officer. Marie throws all custom and manners to the whim and runs to Tonio’s arms. The marchioness reveals that she is, in fact, Marie’s mother and not her aunt and blesses the union of the former la fille du regiment and the officer.