The Simpsons (1989): Season 17, Episode 10

17 - 10: Homer's Paternity Coot

Marge's rage against the new toll road system somehow leads to the discovery of a frozen mailman and his 40-year-old mail. The letters are returned to the rightful recipients, including one to Grandpa Simpson's deceased wife, revealing a secret love affair and the real father of her baby, Homer. 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 his surrogate father of 40 years. But soon Homer realizes that a DNA test doesn't necessarily tell who his true father is. Blackboard 'I am not smarter than the president'.