Nancy Drew embarks on a thrilling quest, searching for a flight of 99 steps to unravel the mystery behind her friend’s peculiar dream. Her pursuit takes her to the enigmatic streets of France. However, before she can even leave the United States, an enigmatic figure, Monsieur Neuf, issues a chilling warning, urging the young sleuth to abandon her mission.
Accompanied by her loyal friends Bess and George, Nancy lands in Paris to reunite with her father, who is engrossed in another perplexing case. His task is to uncover the source of fear that is compelling the wealthy financier Monsieur Leblanc to sell off substantial amounts of securities. As Nancy delves deeper, she stumbles upon startling revelations that suggest a strong link between her father’s case and her own mystery, all tied to the 99 steps. A mysterious man seems to hold a tight grip over Leblanc. Is it blackmail? Nancy’s relentless pursuit of clues leads her to the romantic chateau country in the Loire Valley, where a perilous web of danger begins to close in on the trio of girls.