In a very surprising move on Valentine's Day, Marge allows Homer to have a guys' night out with Bart as her Valentine's Day gift to him while she and Lisa go to a restaurant for dinner. While Homer and Bart have a good time and bond as father and son, Marge finds she has little in common with Lisa and distracts herself at the buffet table. During this, Lisa spots a handsome boy named Nick through a crack in the wall between their tables. The two become attracted to each other and begin dating.
Eventually, Lisa invites Nick over to meet the rest of her family. However, despite being impressed by Nick, Marge warns Lisa not to spend too much time with him, worried that Lisa is becoming too different from her. Confused at this, Lisa goes to Grampa for advice.
Grampa tells her a story about Pyramus and Thisbe, two lovers who would talk through a crack in the wall between their houses as their families hated each other. They kissed underneath a Mulberry tree, signifying their eternal love. Inspired by this, Lisa asks Grampa to drive her and Nick to Mulberry Island by sunset so they can share a moment of eternal love, and he agrees, stealing his retirement home's shuttle van to drive them. However, he drops them off to make the rest of the journey when the police arrest him for stealing the van and the TV remote from the home.
Homer, Marge and Bart bail him out and catch up to Lisa and Nick just as they begin rowing their boat through the river to the tree. Marge uses water shoes to skate across the river, making it to the Mulberry tree where Lisa and Nick are having second thoughts and decide to break up instead, doubting that they could devote the rest of their lives to one another. Marge comforts Lisa when she tells her the moment she wanted, and then kisses her to remind her of their own eternal mother-daughter love.
Meanwhile, Bart and Milhouse are inspired by the show MythCrackers to debunk some urban schoolyard legends. At first, the students are fascinated by the myth debunking. However, this eventually works out too well and the students are left disappointed by the lack of interesting myths in school. So Bart and Milhouse come up with a way to make school fun again by creating a myth where Groundskeeper Willie is a werewolf.