As Marge and Lisa watch television, they see a commercial for the Smoke Damage Outlet Store featuring a cameo appearance by Smokin’ Joe Frazier. Intrigued and excited, Marge decides to check out the store. She and Lisa hop in the car, but on the drive to the store they end up on a toll road that charges 75 cents per vehicle. Marge exits the highway and takes The Ol’ Cheapskate Trail, avoiding the toll. People in the cars behind her decide to follow suit. Mayor Quimby is horrified when he sees what people are doing. He instructs Chief Wiggum to force people to take the toll road. The next time Marge and Lisa approach the toll, they encounter a series of tire spikes preventing them from backing up. Marge realizes the government figured out a way to block her favorite shortcut. When Marge enters the toll area, she refuses to give in. She throws the car in reverse, colliding with the car behind her. This triggers a domino effect, and car after car is thrust backwards over the tire spikes. The punctured tires are transported to the Springfield tire fire, creating an even larger inferno. The heat from the tire fire causes ice on Mount Springfield to melt. Soon after, the body of a mailman is discovered in the melted ice. It is determined that the mailman died many decades earlier. The post office vows to deliver every letter in the forty-year-old mailbag. A mailman delivers a letter to the Simpson house addressed to Mona Simpson. Lisa reads the letter aloud. It was written by a lifeguard who was having an affair with Mona while she was married to Grampa. At the end of the letter the lifeguard wrote, “But I know in my heart the baby you’re carrying is mine.” Marge tells Homer that Grampa may not be his father after all.

Homer grows obsessed with finding out who his real father might be. He decides to perform some research. He makes his way to the local library, where he finds a book called “Springfield Lifeguards of the 20th Century.” Since the letter from Mona’s paramour is signed “M,” Homer is able to pinpoint the author: a British man named Mason Fairbanks. Homer locates Mason, and posing as a reporter for a local newspaper, begins to ask Mason a series of questions. When Mason grows suspicious about Homer’s intentions, Homer hands him the letter he wrote to Mona. When he realizes that Homer could be his son, Mason is elated. Homer responds by kissing him on the lips. At the Simpson house Marge, Bart, and Lisa watch a show on Springfield Public Access Television called The Bully Corner; featuring Jimbo, Dolph, and Kearney. The threesome beat up actor William H. Macy. When Homer returns home he announces that Mason is his real father. Mason takes Homer and his family for a cruise on his boat. Mason shows them a map, and explains that he’s located some valuable treasure lost hundreds of years ago when a Spanish galleon sank on a return voyage from the Caribbean. Homer introduces Mason to Grampa. When Marge sees the affect it has on Grampa, she recommends a paternity test. Homer makes his way to Dr. Hibbert’s office, where Hibbert performs the test. It turns out that Mason is Homer’s biological father.

Marge, Bart, and Lisa try to cheer Grampa by spending time with him at the Springfield Retirement Castle, but they soon grow bored. Mason and Homer set out on a voyage to hunt down the treasure. They climb into their own submersibles, and descend to the bottom of the sea. They eventually find the remains of the Spanish galleon. Using the claws on the submersibles, Homer and Mason each grab hold of a treasure chest. As he makes his way back to the surface, Homer gets stuck on a coral reef. As he attempts to dislodge the vessel, Homer bangs his head… and experiences a series of flashbacks involving his life with Grampa. Homer then passes out. When he reawakens in the hospital, Homer is told he was in a coma for three days. He tells Grampa that while he was unconscious on the ocean floor, he remembered everything Grampa has done for him. Homer says he’s not interested in the DNA tests. To him, Grampa will always be his father. He tells Grampa how much he loves him, and Grampa says the same in return. The two hug. Grampa then announces that he switched the names on the DNA samples. Homer realizes that only a true father would lead his son to believe he belonged to someone else.