"Mother Simpson" is the eighth episode of Season 7.
To get out of cleaning a highway as part of 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 of his workers to clean up a highway that is maintained by his company. His workers are understandably ticked off about this, especially since Mr. Burns grabbed a photo opportunity by posing as a cleanup volunteer then immediately leaving afterwards, 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. But in reality, Homer actually pushed a dummy of himself off of the cliff so that he could get off of work to spend the Saturday by having fun with his family.
The next day, news of Homer's "death" spreads across Springfield, and 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, but she yells, "Get out of here, you ghouls!" at them and slams the door. Marge finally puts her foot down when the house loses power, since the residence is in Homer's name. The following day, Homer goes to the Springfield Hall of Records to get the whole "misunderstanding" sorted out. While 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 what he thinks it is) and discovers that it is actually Walt Whitman's grave. Nearby, he sees his own grave (possibly prepared by Patty and Selma since it has the same name written on it when they showed it to Marge earlier), and falls into it. His mother, Mona Simpson, suddenly arrives at the gravesite and doesn't think that he is actually Homer, but he tells her that it's his grave, so they have a rather emotional reunion after twenty-seven years of separation.
He takes her home to meet the family, which causes quite a stir and shock. She also bonds with Lisa, being on the same intellectual level. However, 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 drivers licenses with different names and Mona's photo, in which Lisa wonders whether she is a con artist or not. Meanwhile, Homer and Marge are wondering why his mom left him alone for twenty-seven years (Homer thinks he must've been a lousy son who no mother could love), and decide to find out. With Marge by his side, 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 returned to Springfield.
The story flashes back to 1969, when she was still with Homer and Grampa. Obviously, she and Grampa didn't 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. Mr. 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 by herself. 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," according to the post office worker).
At the post office, Mr. Burns recognizes her and calls the FBI to send 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, he begs for sex, but Mona refuses. Meanwhile, the FBI and Mr. Burns track her down, through a cab driver, a gravedigger and Patty and Selma.
Homer and Mona escape, thanks to Clancy Wiggum, who is grateful to Mona for curing his ailment, but she has to go underground by herself 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 finishes with Mona driving away while Homer waves into the distance until the van is out of sight. Later, he is seen at night, still sitting on top of his car while gazing at the stars.