Marie-Grace Gardner, a doctor's daughter who has just returned to her native New Orleans in 1853, makes friends with Cčile Rey, whose prosperous family are free people of color, and is persuaded to change places with her at separate Mardi Gras balls.
Marie-Grace can't wait to begin her journey up the Mississippi River with her father. The steamboat they're traveling on is the biggest and fanciest boat Marie-Grace has ever seen. It's crowded with all sorts of interesting passengers, including Wilhelmina Newman, a girl her age. Wilhelmina is traveling alone, and she's carrying a secret in one of her trunks--clues to hidden Gold Rush treasure. Marie-Grace and Wilhelmina have to unravel the clues...
When Marie-Grace discovers a baby outside her father's office and a slave catcher claims the boy, she helps place him at a white orphanage where she becomes a volunteer, as her friendship with Cčile grows and she hears rumors of yellow fever.
Marie-Grace is excited that a well-known English opera company will perform at the very theater where she takes singing lessons from Aunt Ocǎne, but as she and her friend Cčile help out backstage, they make disturbing discoveries.
As the yellow fever epidemic continues to ravage New Orleans and the orphanages become more crowded, Marie-Grace and Cčile help with the orphans, until Marie-Grace learns that her Uncle Luc's fiancě, Mademoiselle Ocǎne, has fallen ill.