Bart, needing money for the new video game console Gamestation 256, takes a job hanging menus on doors for a Thai restaurant. Lisa is concerned that the menus are wasting paper and hurting the environment, but the family ignores her worries. On a trip to Krusty Burger to celebrate Bart's new job, they see protesters dressed as cows on the roof of the restaurant. The protesters unfurl a banner and accuse Krusty Burger of deforesting the rainforest to create grazing land for cattle. The police arrive and shoot the protesters with bean bag rounds. As the protesters are being arrested, Lisa meets their leader—radical environmentalist Jesse Grass—and is instantly smitten with him.

Lisa visits Jesse in jail, but feels intimidated when she sees that he is more dedicated to environmentalism than she is. She attends a meeting of Jesse's activist group, Dirt First, and learns that an ancient tree in Springfield is scheduled for demolition. Jesse asks if anyone in the group would be willing to live in the tree to prevent its destruction. Lisa, hoping to impress him, volunteers. She climbs the tree and sets up camp, but after a few days she begins to miss her family. She sneaks away from the tree at night and goes home to see them, but finds them asleep. She lies down with them and accidentally falls asleep. When she rushes to the tree in the morning, she finds it has collapsed.

Upon returning home, Lisa learns that the tree was not cut down by the loggers, but knocked down by lightning, and that she is presumed dead. When Lisa learns that the forest will be turned into a nature preserve in her honor, she decides not to reveal that she is alive. Homer and Bart immediately begin to take advantage of the sympathy of the townspeople. But when the Rich Texan decides to turn the forest into an amusement park called "Lisa Land" rather than a nature preserve, Lisa reveals that she survived. Jesse Grass cuts down the log that has been turned into a "Lisa Land" sign, and it rolls down a hill and into Springfield's business district. Jesse is jailed again, and the log rolls across the country, passing Mount Rushmore and ultimately winding its way down Lombard Street in San Francisco before reaching the Pacific Ocean and heading out to sea.