Homer fails a National Fatherhood test and admits he is a bad father. Homer feels he needs to build a relationship with Bart. So he helps build a soapbox racer for the up-coming local derby.
Bart does badly in the qualifying race, and after Martin Prince gets injured, he asks Bart to take over and use his soapbox racer so they can beat Nelson. Homer is very upset by this, and refuses to watch Bart race.
However, seeing the fatherhood test on the fridge, he answers all the questions and realizes he is the perfect father. Homer attends the derby to cheer Bart on. Bart beats Nelson and wins the Soapbox derby!