Eight-year-old Mackenzie Goode has a sense of humor that always gets her into trouble. But in third grade, she makes the ultimate promise to herself: no more mistakes. Mistakes are for babies and the last huge mistake she made, back in second grade, caused her a forever grounding and to see a look in her parents' eyes she never wants to see again: disappointment. When the principal of the school visits Mac's class as a mystery reader, Mackenzie's will is tested. Instead of listening to the principal read, Mac sings her best friend, Cheese, a song she makes up about their friendship. Right in the middle of class! Mac winds up in the worst sort of trouble! The see-you-after-school kind of trouble. The grounded-for-life kind of trouble. The kind of trouble that is sure to create that look in her parents' eyes she was trying so hard to avoid. Fixing this sort of mistake won't be easy, but with the help of Cheese (and her parents) Mac tries.