To get out of cleaning a part as community service, Homer fakes his own death. When this results in the family's utilities being cut off, Marge puts pressure on him to reveal that he is alive. However, the spurious news of his "death" brings his long-lost mother - a hippie who is on the run from the law because of her activism against Mr. Burns - back to Springfield.
On a beautiful Saturday, Mr. Burns has got all his workers to clean up a highway maintained by his company. His workers are understandably ticked off about this, especially since Mr. Burns grabs a photo opportunity by posing as a cleanup volunteer then immediately leaving, showing his failure to lead by example. Homer seems to be missing, but he shows up on top of a nearby cliff. On his way down (to work), he slips and falls to his death, much to the shock of his co-workers. In reality, Homer pushed a dummy in his likeness off the cliff so that he could spend the Saturday having fun with his family.
The next day, news of Homer's "death" spreads and soon Marge starts getting condolences from prominent Springfieldians. Patty and Selma offer their condolences in the form of a tombstone with Homer's name on it, which they have been saving since Marge's wedding day; she yells at them "Get out of here, you ghouls!" and slams the door. Marge finally puts her foot down when the house loses power, since the residence is in Homer's name. The next day, Homer goes to the Springfield Hall of Records to get the whole "misunderstanding" sorted out. There, after his problem is taken care of, he sees in the record that his mother is still alive. Incredulous, he goes to his mother's grave (or so he thinks it is) and discovers that it is Walt Whitman's. Nearby, he sees his own grave (possibly also prepared by Patty and Selma), and falls into it. His mother (Mona Simpson) arrives suddenly at the gravesite and doesn't think he is actually Homer but he tells her it's his grave, and they have a rather emotional reunion after 27 years apart.
He takes her home to meet the family, which causes quite a stir. She bonds with Lisa, being on the same intellectual level. Suddenly, a police car drives by and Mona runs inside the house, making Lisa a little suspicious. She shares her suspicions with Bart, who had raided Mona's purse and found several driver's licenses with different names and Mona's photo. Lisa wonders whether Grandma is a con artist. Meanwhile, Homer and Marge are wondering why his mom left him for 27 years (Homer thinking he must've been a lousy son who no mother could love), and decide to find out. In the next scene, they go downstairs to Mona, and at the same time, Bart and Lisa come up from the basement to her. The whole family then decides to interrogate Mona about her shady past. She decides to tell them the truth after the family threatens to tell Grampa that she's returned.
The story flashes back to 1969, when she was still with Homer and Grampa. Obviously, she and Grampa do not share the same interests. After being radicalized by Joe Namath's hair during the telecast of Super Bowl III, she joins a group of hippies who protest Mr. Burns' germ warfare lab. They detonate an "antibiotic bomb" inside the lab, killing all the germs and curing lab security guard Clancy Wiggum's asthma. Burns, angry about the destruction of his "precious germs," manages to identify Mona as one of the perpetrators of the deed after she stopped to help Burns up after being trampled by the group. Mona then went into hiding. Upon hearing about these events, Homer wonders why Mona never sent any care packages. Mona, insisting that she did send such packages, heads to the post office to claim the undelivered packages ("That's what happens when you don't tip your letter carrier at Christmas," the postal worker explains).
At the post office, Burns recognizes her and calls the FBI and gets sent Bill Gannon (voiced by Harry Morgan) and Joe Friday (of Dragnet fame). At the Simpson home, Grampa gets the shock of his life when he visits and meets Mona. After a tense exchange, Abe begs for sex. Meanwhile, the FBI and Burns track her down, through a cab driver, a gravedigger and Patty and Selma.
Homer and Mona escape, thanks to Wiggum, who is grateful to Mona for curing his ailment. She has to go underground again. One final tidbit proves that Mona is indeed Homer's mother: as she bumps her head while getting into the hippies' van, she cries out "D'oh!"
The episode ends with Mona driving away while Homer waves into the distance until she is out of sight. He is seen at night still there, sitting on his car and gazing at the stars.