As Marge goes shopping, a toll booth appears, but Marge and other Springfield residents take "The Ol' Cheapskate Trail". Mayor Quimby enforces tire spikes and blocks off the escape route because he desperately needs the money in order to “de-python” the town fountain. The next time Marge comes up to the booth, she backs up, causing many cars' tires to become severely damaged. The tires are thrown in the tire fire, melting ice on Mount Springfield and revealing a mailman which has been frozen for 40 years. His latters contain many revelations (Dewey Largo never knew he was accepted into a prestigious music program, and Moe never had an allergy that kept him inside for most of his childhood) and one of the forty-year-old letters is delivered to Homer's mother, Mona Simpson. It is from her old lifeguard boyfriend, whose name begins with an M, who writes that if Mona replies to the letter, she has chosen him, and if she does not, she is choosing to stick with her husband, Abe. He also writes that either way, in his heart he knows that the baby she is carrying is his.
Homer goes to the library to find his supposed father by looking in "Lifeguards of Springfield in the Twentieth Century." The only person in there whose name begins with M is Mason Fairbanks. Homer goes to his relatively large house pretending to be a reporter. He tells him that he is interviewing people who wear sweaters, and he is let inside. Mason shows him around his home, and shows him his ship, The Son I Never Knew. Homer eventually tells him that he thinks that he is his father, and Mason is delighted. He takes Homer and the rest of the family, excluding Abe, on a ride on his ship and tells them the story of the lost emerald treasure of Piso Mojado, which impresses the family. However, when they come home, Grampa angrily accuses Mason of having tried to steal his wife and now trying to steal his family, and is deeply saddened that Homer would even think that Mason could be his real father. They decide to have a DNA test, and after a suspenseful wait, Homer is thrilled to learn that his real father is Mason Fairbanks.
While Marge, Bart, Lisa and Maggie are having an awkward, uneventful visit with Grampa, Mason and Homer are underwater (which is terribly polluted by nuclear waste) in their own individual submarines looking for the lost treasure. Homer gets separated from Mason, and he follows a small light that he thinks is him. It is actually a glowing fish, and Homer gets stuck in some coral. As his oxygen begins to run out and he starts to lose consciousness, he sees poignant flashbacks of himself and Abe. They show Abe entertaining a child Homer, praising teenage Homer's (poorly drawn) picture of Abe, and supplying for Homer's marriage some money (which a bird subsequently steals, leading Abe to sadly say, "I gave you all I had, and it still wasn't enough.") When his last bit of oxygen runs out, the last thing he sees is Mason heading towards him. When he wakes up, he is in the hospital, and Bart tells him that he has been in a coma for three days. Abe starts to leave, but Homer grabs his arm and asks Mason, Marge, and the kids to leave. He tells Abe of his memories and says he doesn't care what the DNA test says--he still considers Abe his real father. Abe then tells a shocked Homer that he is actually his father; he switched the labels on the DNA samples after seeing how happy Homer was with Mason, and the episode ends with the fully confirmed biological father-son duo sharing a hug.