Fourth-grader Katie is concerned when an older girl uses live mice in a science fair project, but when Katie turns into the sixth-grader and has to run the experiment herself, she has a different opinion of the student. Katie can't wait for the school science fair. But when she hears that a sixth-grader named Selena is using her pet mice as part of a science experiment, Katie does her best to put a stop to it. When she turns into Selena on the day of the fair, she gets her chance to set the mice free. What a disaster! How will Katie eek her way out of this one?