The Simpsons: Season 17, Episode 10
The result of Marge's anger against Quimby’s new toll road leads to the discovery of a frozen mailman and his 40-year-old mail. The old mail is delivered to the recipients, including one to Grampa Simpson's deceased wife, and it reveals a secret love affair and who the real father of her baby Homer might be. Confused and emotional, Homer finds Mason Fairbanks, his wealthy and much improved paternal dad, and happily establishes a father-son relationship with him while rejecting Abe, his surrogate father of 40 years, especially after a DNA test reveals that Mason is indeed his real father. Homer joins Mason on an undersea treasure hunt and when Homer’s life is in mortal danger he reflects upon his childhood and realizes who his true father is. When he confesses that to Abe, Abe has a confession of his own.