Ace has built her own go-kart, but when she is ready for a test run it will not start, and she has to figure out what part is missing--and then in the middle of the race something else goes wrong.

For Enzo, the Fourth of July means three things: fireworks, family, and car races! This year Zoë is finally old enough to compete in the Kids' Kart Challenge. But when one of the boys at the track says that girls aren't fast drivers, Zoë gets nervous and backs out of the race. Enzo knows it's up to him to help his family regain the confidence to compete on the big day.