One lovely summer night, fifteen-year-old Melanie went out into the garden in a nightgown and, climbing a tree, began to admire the ball that her aristocratic neighbor was throwing. Having fallen from the tree, she fell into the hands of a handsome man. Melanie didn't just twist her ankle: when she saw the thin mustache and mocking gaze of the seductive Marquis de Varennes, she lost her heart. But love is evil... and Melanie has not yet been given the opportunity to know what dangers await her in the near future. It all starts in the compartment of the Mediterranean Express, where the love affair she was expecting unexpectedly turns into a cool detective story, in which the spy artist Antoine Laurent and the express conductor Pierre Beau will become her reliable partners. Meanwhile, there was her triumph in a Lanvin dress in the opera box and the interest shown in her beauty by the king...