As the yellow fever epidemic continues to ravage New Orleans and the orphanages become more crowded, Marie-Grace and Cécile help with the orphans, until Marie-Grace learns that her Uncle Luc's fiancée, Mademoiselle Océane, has fallen ill.

"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...