Austin is building his first Soap Box Derby race car. His sister, Bridget, is being a pest and wants to know everything because next year she's going to race. Austin says there are more guys than girls in the Derby. Bridget wanders next door where Alex is working on his old car. Alex suggests she do some research to find out the number of girls and boys in the competition and the ratio of wins.

Bridget, Austin and Kevin are playing in the back yard. As Bridget goes down a sliding board she asks why she can't slide up the slide. Austin thinks that's a silly question: it's gravity! Bridget asks for an explanation of gravity, which Austin and Kevin can't supply. Kevin does remember that the Soap Box Derby is called the Gravity Grand Prix. With that they decide Alex should know the answer.

Austin is having trouble interpreting the plans for his Soap Box Derby car. The plans use scale drawings and he's having trouble understanding how items on the drawing relate to the actual car. Bridget can't help so they go next door to ask Alex. He shows them a collectors model of a Soap Box Derby car and compares it to the one in his garage. By comparing the two, he explains proportion.

Bridget and Austin are on their skateboards when Austin announces that he learned he has more potential than Bridget does he'll go farther in the world than she will. To demonstrate, he challenges her to a skateboard race. They start at the same time, but Austin crosses the finish line ahead of Bridget. He says this proves it. Bridget is skeptical and thinks something is fishy.

Austin test-drives his partially completed car and it doesn't go very straight. He can't figure out why. Bridget has been studying shapes in school and notices the triangles on the plans and the ones on the car made by the axles aren't the same. Austin says, "Close enough!" They argue about the importance of Bridget's observation and go to Alex to settle the argument.

Bridget and Austin go with Alex for a visit to the All-American Soap Box Derby headquarters in Akron, OH. On the way, Bridget sticks her hand out the window and holds it against the wind and turns it to cut the wind. Alex tells her she is experiencing the friction wind causes, just like in the Soap Box Derby. Whey they get to Derby Downs, they look at some of the old cars and compare their shapes.

Bridget and Austin are on their way home from school whey they see their new neighbor getting out of a vehicle in a wheelchair. As she prepares to go into her house, Bridget and Austin offer to help. "Why thank you," she says, "but that's OK because I have wheels and an axle, and a ramp on the front of my house to give myself a mechanical advantage."

Austin, looking at statistics about the Soap Box Derby, discovers that racers used to start higher up the hill. He wonders why this was changed.