Katie loves skating with the Wheelas and the fact that she's officially a superhero sidekick. But now that school's starting, everything's changing. The Mousetress is getting blamed for things Katie knows she didn't do. Sidekick training is NOT as exciting as she'd hoped. Katie's best friend Beth is back in town and Beth's new boyfriend is always hanging around (ugh!). Not to mention that all of Katie's friends are mad at her. Fixing this will be harder than any skateboarding trick. But with the help of 217 slightly out of the ordinary cats, Katie's going to try! Can she clear the Mousestress's name, uncover the real supervillain, and become the sidekick (and the friend) she's always dreamed of being?