If getting your best friends to stop fighting and dealing with Margaret wasn't hard enough, now Elizabeth is faced with the task of trying to find out who the Wilson Middle School thief is. When things around school start disappearing, Elizabeth finds herself in the position of becoming a detective. She rounds up her friends to tell them her plan to catch the thief. But it's really Pappus' plan. And, like all Pappus' plans, it's a good one. Elizabeth has her doubts though that it will work. It seems the thief is just too smart. When Elizabeth discovers the thief is planning to steal money from the school book fair, it becomes a race against time for Elizabeth and her friends to find out who the thief is before the day of the fair. Then there's the mystery of the house across the street. What's with that house anyway? How can Elizabeth possibly solve both mysteries?