Gabby Gomez is back from vacation and all is normal at school . . . except that Ms. Smoot is missing and Mr. Hansen, the substitute teacher, is acting a little bit nutty. As the day spins on, Mr. Hansen gets nuttier and nuttier . . . and furrier? When Mr. Hansen suddenly turns full-on hamster, Gum Girl needs everyone’s help-Ninja Rina, Natalie, and even Robo Chef-to help Mr. Hansen (and save the city)!