10-year-old Lucy is actually too good for this world. She is well-behaved, kind and absolutely correct in every situation. Her best friend Rima and she herself want to make the world a little better with every little action. But then disaster threatens: Lucy's parents run an ice cream parlour, and when the expensive ice cream machine breaks down, the parlour is threatened with closure. Lucy wants to prevent this at all costs. However, after much deliberation, she sees only one way to get money quickly: a bank robbery. To prepare for her big coup, she makes a deal with the class crook Tristan. Under the code name "Operation Lucyfer", he is to teach her how to be mean and ruthless. So Lucy gets tutoring in stealing, lying, cheating and blackmailing - and proves to be an astonishingly docile pupil. Lucy hardly recognises herself. However, her transformation from overachiever to crook causes many a problem.