Homer takes a fatherhood quiz and discovers he knows next to nothing about his son. He signs up for therapy at the National Fatherhood Institute and after a confidence-building pep talk, offers to help Bart build his own soapbox racer, which they dub "Li'l Lightnin'", for the upcoming Soap Box Derby. Bart is reluctant to want his father to help him, concerned that he may cost him the race. However, Homer refuses to hear any of it, insisting to help him out. At the qualifying race Bart and Martin, who has built a high tech, aerodynamically superior racer, form an alliance vowing that either must somehow beat bully Nelson and his very intimidating racer, the "Roadkill 2000", "armed with every dirty trick in the book." As the race starts, Bart can barely reach any kind of noticeable speed with Li'l Lightnin' and the car falls apart before reaching the finish line.
Martin's car wins the race. but was built too aerodynamically precise and the steepness of the hill causes it to careen past the finish line and crashes. Martin suffers a fractured arm in the accident and thus cannot race against Nelson, who finished second. Martin offers Bart the seat, and even though Bart believes Homer will be devastated if he abandons the racer they built, he accepts the offer as there is no other way he can race and win.
As Bart feared, Homer is bitterly disappointed over his decision. Homer is so upset that he even refuses to go to the final race to support Bart, insulting him and Martin and acting spiteful toward Bart and the rest of the family. Troy McClure starts crying too. Marge decides that she has had enough of Homer's surly attitude and confronts him, by telling Homer that she defended him when people, including her sisters Patty and Selma, called him an incompetent father. However, she states that his latest behavior proved he is being selfish and if this is how Homer wants to act on such an important day for Bart, he really is a bad father, a statement that only makes Homer feel worse. Maggie tries to cheer Bart up but he ended up calling Los Angeles.
As Bart gets ready to race in the final match with Martin's newly tuned racer, Homer thinks to himself about how wrong he has been. After retaking the fatherhood quiz again, he realizes that Bart needs his support regardless of whose racer he is using and he rushes off to the race. At the starting line, Homer wishes Bart good luck and tells him that no matter how the race ends he will still be proud of him. The race is tough as Nelson pulls every dirty trick in his arsenal, but through his skill Bart finishes first and the team enjoys their victory.