"The Way We Weren't" is the 20th episode of Season 15.
When Homer admits to Marge that she was not his first kiss, we flashback to Homer at summer camp.
Bart tricks Milhouse and Ralph into doing chores in the front yard. Sherri and Terri appear and introduce them to their "boy-crazy" cousin who thinks Bart is cute and they go to play spin the bottle in Bart's treehouse. Milhouse gets to kiss the girl, but she backs away and he accidentally kisses Homer instead. Homer strangles Bart over the use of a beer bottle Milhouse wanted to use (the beer actually landed "safely" into Homer's mouth, who drank it while napping), it lands them in the Simpson family court, with Lisa presiding as judge.
Marge, giving a testimony, tells the kids that when she was in high school, she had her first kiss with Homer, but Homer confesses that it was not his first kiss, giving Marge a huge shock. Homer recalls that when he was 10 years old, he went to a camp for underprivileged boys, Camp See-A-Tree, where Homer meets Lenny, Carl, and as a disgruntled counselor, future tavern owner Moe. It turns out the summer camp is more like a prison and they go to work in the kitchen at a rich girls' camp across the lake. Homer finds a retainer and returns it to the girl who lost it, though he cannot see her (they are separated by the kitchen wall). She wants Homer to see her later that night. He does so, although due to an accident with a switchblade he is wearing an eye patch. Homer tells Bart and Lisa that she was the prettiest girl he had ever met until he later met their mother. Marge admits that she was that same girl Homer met, and if she'd known that he was that boy all those years ago then she never would have married him.
Marge gives her side of the story, explaining that she was with her sisters Patty and Selma, as well as Luann (future Mrs. Van Houten), Helen (future Helen Lovejoy) and Cookie Kwan at "Camp Land-A-Man". At camp, Marge burned her hair one night with an iron by accident, and had to dye her normally blue hair brown, which is why Homer did not later recognize her in high school. She meets Homer that night (who gives her a false name, Elvis Jagger Abdul-Jabbar) and they kiss, both fantasizing to the tune of "Happy Together" by The Turtles. They agree that the following night they will meet again, but Homer does not come. Marge mentions that she could not trust another boy for years.
Homer says he did not return for another date because he fell off a cliff into the lake and drifted to a fat camp, "Camp Flab-Away", which counted the future Mayor Quimby, Clancy Wiggum (future Chief Wiggum) and Jeffery Albertson (Jeffrey Albertson) among its participants. The directors refused to believe he wasn't attending the camp and strapped him into an exercise belt machine for several hours. Homer escaped the fat camp and made his way to Marge's camp, Marge had left the camp only seconds before he arrived and her sisters, who disliked him even back then, lied to Homer that Marge hated him. Homer said that what they had was special. Patty kissed Homer and asked sarcastically if that was special, to which Homer replies no. Marge realizes the truth and forgives him, but she thinks that Homer won't be able to preserve their love for long. Homer disproves it by showing his half of the heart-shaped stone, which he has kept through all the intervening years. Marge shows that she also kept her half (albeit for different reasons), and the two kiss until the light fades, while the two halves of the stone come together.