Marge gives Homer a test from the National Fatherhood Institute to evaluate his performance as a father. Homer fails the test and determines that he is not a good father. He signs up for therapy at the Institute and after a confidence-building pep talk, offers to help Bart build his soapbox racer. Bart reluctantly accepts Homer's help and together they build a meager racer.

At the qualifying race, Bart's car does not even cross the finish line. Martin's sleek, aerodynamic model easily wins, but loses control at breakneck speed and crashes. Martin breaks his arm and cannot enter the final race against Nelson. He asks Bart to race for him, and Bart gladly accepts. However, Homer feels betrayed by Bart's choice to drive Martin's car instead of the one he helped Bart make, and pouts around the house. Despite being behind the wheel of Martin's machine, Bart does not have his heart in the race.

Homer takes the fatherhood test again. This time, his experience helping Bart build a racer enables him to answer all of the questions correctly. He rushes to the track to watch Bart. Bart sees him cheering in the bleachers and, buoyed by his father's support, blasts by Nelson to win the race.