Twelve-year-old Modesty "Mo" Seto dreams of being a taekwondo champion. Even though her mom disapproves, Mo can always count on her dad, who is her number one fan and biggest supporter. Lately, Mo has been on a losing streak, and it doesn't help that she keeps losing to her archnemesis, Dax, who's much bigger than her. If only she were faster, stronger, not so petite. Mo can't even lean on her dad like usual with how distracted he's been lately.

When Mo learns about the chance to audition to star alongside her idol and legendary martial artist and movie star Cody Kwok, she knows this her chance to prove to her dad, to the world, and to herself that she can compete with anyone, no matter her size. Unfortunately, Dax is auditioning, too. As Mo and her nemesis progress to callbacks, someone attempts to sabotage the movie set and Mo's dad disappears--and both events seem linked to a mysterious book, the Book of Joy.

The book contains information on Xiaoxi Fu, a secret dance-like martial art developed by Mo's ancestral grandmother. Armed with these secret moves and an unexpected ally, Mo embarks on a high-octane adventure to rescue her father, save the movie, and discover an unexpected joy in being small.