It's springtime in New York, and Bea can't wait for her cousin Céline and Aunt Juliette to visit. The girls are looking forward to a fun week of sightseeing, while Aunt Juliette is there to write a profile on a famous French actress, Eva, who's starring in a stage adaptation of Agatha Christie's The Mystery of the Blue Train.

But when the priceless diamond necklace Eva plans to wear on opening night is stolen from her hotel suite, Bea and Céline find themselves once again at the center of a mystery.