In this episode, Homer discovers that Abe might not be his father, and his real father might be a man named Mason Fairbanks. So he tracks Mason down to learn about him, and he really bonds with him. Abe sees this and so he rigs a paternity test to make Homer happy and think Mason really is his dad. In the end the truth is revealed and Abe really is Homer's Dad.
I'm glad that Abe really is his father, otherwise the whole series up to this point would have been a lie. The episode was pretty good from what I remember