Saturdays of Thunder is the ninth episode of Season 3. It features cultural references to films such as Ben-Hur, Lethal Weapon, and Days of Thunder.
Bart becomes deeply immersed in competitive soapbox racing while Homer begins to realize that he is not as good a father as he thought. After attending a lecture at the National Fatherhood Institute, Homer becomes a more attentive father, helping Bart build the ultimate soapbox racer. But Bart chooses not to use it in his big race against Nelson, instead opting to borrow Martin's scientifically engineered aerodynamic racer.
When the local Soap Box Derby is held in Springfield, Bart has his heart set on first prize. But the catch is that he must build his own soap box racer. In the meantime, after taking a fatherhood quiz, Homer discovers he understands next to nothing about his son. He goes to the National Fatherhood Institute, at the institution they give a book on fatherhood. He offers to help his son by building the racer together, and Bart eventually agrees. Unfortunately, Homer's less than adept building skills gives Bart a very shoddy, rickety piece of junk for a racer (named Li'l Lightnin'). This is especially so in comparison to Martin Prince's personally designed and built space shuttle-like racer, Martin B-57. Martin and Bart form an alliance vowing that either must somehow beat Nelson and his very intimidating racer, the MIG-17, armed with every dirty trick in the book.
When the race is underway, Bart can barely reach any kind of noticeable speed with Homer's racer and becomes a laughingstock for it. Meanwhile, Martin has difficulty controlling his racer, as it is moving too fast, which eventually causes his racer to smoke-bust and burn. After getting injured, Martin finds that he can no longer race and opts for Bart and himself to combine forces by having Bart race Martin's racer over the racer he and Homer built. Bart seeing no other way to win agrees, which breaks Homer's heart. When Bart later apologizes and desires Homer's best wishes, Homer selfishly denounces both Bart and Martin, telling Bart to do whatever he likes. Dejected, Bart gets ready to race in the final match with Martin's newly tuned racer. Having seen too much of Homer's behavior towards Bart, Marge confronts him and expresses her disappointment in him. She is quick to remind Homer of the times she defended him in spite of everyone including her sisters, Patty and Selma, telling her that he is a terrible and incompetent father. Marge points out that his latest action in rejecting Bart's choice to be a racer for Martin proves that he is a bad father himself and if this is how Homer would rather behave himself, she tells him off that he had indeed proved her wrong and everyone else right about him being a horrible father. This statement makes Homer feel worse and he quickly tells Marge to leave him alone with his depression.
At the house, Homer thinks to himself about his selfish behavior, and realizing he has been learning a lot about Bart, he goes straight to the fridge in the kitchen and reads the test that came from the institution. He does the test again and passes. He rushes off to the race to show his support to Bart. At the starting line, Homer wishes Bart luck and tells the boy that no matter how the race turns out he will still be proud of him. The race is tough as Nelson pulls every dirty trick in his arsenal, but eventually through his skill Bart comes out on top and the "team" enjoy their victory. Bart also meets his new hero, three time soap box champ Ronny Beck, who hands Bart his trophy. Bart and Homer act like sore winners, which embarrasses Marge who reminds them they should be good winners. Bart then protests that this is the first time he won something and he may never do it again. Bart shares a hug as Homer finally becomes a good father to his son.