"Lady Bouvier's Lover" is the twenty-first episode of Season 5.
Grampa Simpson falls in love with Marge's mother, Jacqueline, but Mr. Burns whisks her away from him. Grampa is broken hearted when learning Jackie is going to marry Burns.
Maggie is celebrating her first birthday, and the rest of the family invites Abe, Patty and Selma, and Jackie Bouvier. At the party, Abe is feeling lonely, and when he is bored, Marge suggests she set up a date for him with her mother--Marge seems greatful with Abe dating her mother while Homer is against it, mostly because if Abe and Jackie get married, it would make Homer and Marge stepsiblings. Abe and Jackie's relationship goes well until when on a night on the town, Jackie goes dancing with Mr. Burns, whom she finds to be more dashing.
Meanwhile, Bart hears about an Itchy & Scratchy animation cel which costs $350 when he is watching a TV shopping channel. He buys one with one of Homer's credit cards. He then endures a long and painful wait for its delivery on delivery day (both figuratively and literally, as Bart had to endure punches in the face from irate townspeople for things that Homer did while waiting for the delivery), before acquiring it, which to his dismay was just Scratchy's severed arm. Burns declares that he is in love with Mrs. Bouvier, against Marge's interests. Bart gets $350 from Burns to pay Homer back (which Homer, not listening to Bart explicitly saying it was to pay him back, thinks Bart is just giving him free money and runs off to spend it foolishly).
On the day of the wedding, Mr. Burns and Jacqueline attempt to marry. Only minutes before Mrs. Bouvier becomes Mrs. Montgomery Burns, in a scene similar to one from The Graduate, Abe crashes the ceremony by arguing that Mrs. Bouvier should not get married to Burns. She declares that she does not want to marry either man, and they hop on a bus and look anxious about their future. This likely leaves Mr. Burns heartbroken.