After Bart and Lisa discover Marge's diploma from Springfield University, Homer and Marge recount one of the darkest points in their relationship, when Marge fell in love with a university professor, Stefane August.
The Simpson family is freezing inside their house since Homer has not paid the heating bill. Bart and Lisa search through an empty box for items to make the fire hotter, and discover a diploma belonging to Marge from Springfield University. Homer and Marge look shocked to find it, and claim it was from their dating years. Lisa does some calculations (taking into consideration how Bart's ten and both of their parents are almost forty), and the results make her and her brother realize that Homer and Marge didn't get married and have Bart until at a much later point in their relationship than what they were initially told. Marge and Homer proceed to describe one of the darker points of their relationship: the 1990s.
In the flashback, Homer and Marge are happily dating, and are currently living in an apartment at Springfield Place, Marge is an avid reader, and Homer is a part of an R&B group alongside Lenny, Carl and Lou. One morning, Marge wakes up to find out she had been accepted into Springfield University, Homer also ends up shocked that she's been admitted, and asks why she didn't tell him (she actually did tell him, but he misinterpreted her discussion of whether she should apply herself to a college with whether she should apply herself to a collage, the latter of which he was against.) but is shocked to learn of the high cost of tuition. Homer, taking pity on Marge, settles on quitting his dream of becoming a musician and instead decides to work at his dad's popular laser tag warehouse, where he is abused by both the children and his dad. At Springfield University, Marge is impressed with her surroundings and with the rebellious professor Stefane August, despite Homer's disapproval.
In the present, the repair man arrives and fixes the heater (asking if they wanted to hear the story of his lost love, Marge continues on with the story). Marge soon begins to admire August, and while caressing Homer after his long day at work, realizes she has feelings for her professor. Marge starts talking to Professor August, who has also fallen for her. August begins manipulating Marge by telling her Homer is a simple "townie" who would not appreciate her intellect. A shocked Homer arrives and catches the two together. In his anger, he reforms his R&B group with a new sound called grunge. His band is renamed to "Sadgasm", and they sing a song called "Politically Incorrect." An angry Marge and Homer soon call their relationship quits, and Marge leaves to go with Professor August.
Homer goes to Moe, who at this point owns a cigar bar. Receiving no help from Moe, Homer goes on to perform a new song called "Shave Me," which causes him to become so famous that "Weird Al" Yankovic parodies his song, calling it "Brain Freeze." Marge finds Homer's music unnerving. Marge and August kiss, it is August's first kiss, and Marge's second kiss. When running onto the beach, August reveals he and Marge share two very different views on marriage. After the two argue, Marge breaks up with him, breaking his heart. A miserable Marge watches television and is surprised to see Homer made a song dedicated to her, called "Margerine" . A special news report with Kurt Loder interrupts, revealing Sadgasm had broken up and Homer is holed up in his mansion, miserable. Arriving there, Marge thinks Homer had been doing drugs and soon begins caring for him. It turns out the needles were insulin for his diabetes. The two soon re-unite. Marge reveals to Bart and Lisa that she learned "Homie is where my heart is". Homer then says that when they were together again, he was too happy to grunge again. While they kiss, August walking his dog outside sees it and says: "Townies."
An estimated 7.58 million viewers tuned in to the episode. Richard Keller of TV squad enjoyed the many cultural references to the 1990s, but felt disappointed that the episode changed The Simpsons continuity. Robert Canning of IGN strongly disliked the episode, also feeling that the continuity change was not a good choice. He said, "What 'That '90s Show' did was neither cool nor interesting. Instead, it insulted lifelong Simpsons fans everywhere. With this episode, the writers chose to change the history of the Simpson family." He gave the episode a 3/10, and suggested that this episode should have been set a decade earlier to fit classic Simpsons continuity.