It's Paris, 1911. Millie Falaise is the daughter of the head of security at the Louvre Museum. While he guards the paintings and statues, Millie enjoys the museum as her playground. When the Mona Lisa, the most famous and valuable painting in the world is stolen, Millie's father is frantic, and the Louvre goes into turmoil. Millie decides to get to the bottom of this, but who could be responsible for such a mystifying crime?