Mr. Burns has all of his employees clean up a highway maintained by his company. Not wishing to waste his Saturday, Homer fakes his death so that he does not have to take part. He uses a replica of himself and throws it into a waterfall which gets severely broken and eventually goes into the turbines to which everyone thinks Homer killed himself. The next day, news of Homer's "death" spreads, and after getting many flowers and sympathy cards, as well as a tombstone, Marge finds out and orders Homer to go to the Springfield Hall of Records to get the "misunderstanding" sorted out. While sorting out the problem, Homer gets into an argument with a clerk who claims that Homer's mother is still alive, in spite of Homer's belief that she died while he was young. Homer visits what he believes is her grave, only to discover that it belongs to Walt Whitman. Nearby, he sees his own grave and falls into it. A woman approaches and chastises Homer for falling into her son's grave. Homer realizes that the woman is his mother, Mona, and the two have an emotional reunion after 27 years apart.
Homer takes Mona home to meet the family and she bonds with Lisa, being on the same intellectual level. While the two are sitting on the front steps, a police car drives by and Mona runs inside the house, making Lisa suspicious. She shares her suspicions with Bart, who had raided Mona's purse and found several driver's licenses with different names; Lisa suspects that Mona is a con artist. Meanwhile, Homer and Marge are wondering why his mother left him for 27 years and the two decide to confront Mona, who decides to tell them the truth.
In a flashback to 1969, Mona is a housewife who still lives with Homer's father Abe. While Abe is watching the 1969 Super Bowl, Mona becomes inspired by Joe Namath's long hair. She joins a group of hippies who protest Mr. Burns' germ warfare laboratory, who is deliberately trying to poison everyone in Springfield. They detonate an "antibiotic bomb" inside the lab, killing all the germs and curing lab security guard Clancy Wiggum's asthma. Angry about the destruction of his "precious germs", Burns runs to the lab, but is trampled by the fleeing hippies. Mona goes back to help him, and Burns manages to identify her as one of the perpetrators, forcing Mona to leave her family and go into hiding.
Mona reveals that she sent Homer a care package every week, although he never received them, and they go to the post office to claim them. There, Burns recognizes her and calls the FBI. The FBI and Burns manage to track Mona down and invade the Simpson home. However, Homer and Mona manage to escape thanks to a tip-off from Wiggum, who is grateful to Mona for curing his asthma, and thereby allowing him to join the police force. Mona is once again forced to go into hiding and she and Homer say goodbye. After Mona leaves, Homer remains in the night, sitting on his car and watching the stars.