karen is obsessed with "solving mysteries" around her dad's house. She and her brother together solved mysteries about missing glasses, missing cookies, missing books and so forth but after karen, nancy and hannie start a detective agency and her brother is upset he wasn't asked to join, her pet rat disappears and she's worrying so much searching for her all over the place. Also their christmas lights in the front yard keep disappearing and karen solves that mystery too, reading about how she did that was the most exciting part of the book. And when she finds out where her rat was the whole time it's really shocking, read to find out what happened. The activities in the back of the book are great too. There are puzzles about cracking codes and logic puzzles and instructions on how to build magnifying glasses and doing some mysterious science experiments and things like that, finding out the answers are like solving mysteries too.