It’s been more than four months since Jenny, Molly and Lora solved the case involving the puppies. It seems they have forgotten all about the dangerous Belarusian men… almost. When they hear through Lora’s mom that a painting in the school library has been vandalized, they cannot resist. 
It turns out a new friend could be in real trouble -- at school and at home. It also turns out that it’s not just any painting, but a version of the Mona Lisa that many believe was painted by DaVinci himself! The story follows the girls as they stay one step ahead of the grown-ups and solve The Case of Mona Lisa’s Mustache -- saving their friend along the way.