When Melody and Heidi Valentine skip school on Wednesdays, it's not to goof off. The sisters own and operate their own doll factory -- although maybe "doll" is not the most accurate word. The two-foot tall robots are actually computerized miniature androids, designed to simulate real-world professionals. There's the Tennis Instructor, the Policeman, the Math Teacher, the Banker, Ballerina, Actress, Nurse, Farmer, and many others, programmed to help young people learn more about a career they might be interested in pursuing. Then one day the Tennis Instructor decides to tamper with a girl's stereo, and Heidi and Melody realize that someone has been corrupting their dolls. But who? And why? With the help of their friends, Chris and Ryan Warner, the girls delve into an internal investigation that reveals the frightening truth. This adventure takes place in one day, from the time the girls board the special purple bus that takes them to the factory, to the moment they face their nemesis in a showdown that only one side can win.