Lisa finds herself ridiculed by her meat-loving father and school friends when she decides to become a vegetarian after befriending the lambs at a petting zoo. However, with the help of Apu and Paul and Linda McCartney, she learns to embrace her newfound vegetarianism while also respecting the choices of others.

When a 150-foot whale beaches itself, Lisa names it Bluella and steps in to care for it, staying by its side and reading poetry to it at night. The whale sadly dies, but Lisa manages to save its two calves from sharks. Bluella was later referenced by Mayor Quimby as one example of Lisa's many "annoying" environmental pleas.

After falling for a handsome activist, Lisa joins his environmental crusade and volunteers to live in the branches of an ancient tree in Springfield which is scheduled to be cut down. But things don't go according to plan when the tree is struck by lightning and the Rich Texan decides to build a theme park in the forest. Lisa saves the day, of course, after leading a vocal protest.

Lisa takes it upon herself to help Mr. Burns win back his fortune through environmental means. Inspired, he creates the "Little Lisa Recycling Plant," although he is actually using recycled plastic to empty the sea and sell the animal meat as slurry. Mr. Burns offers 10 percent of the $120 million plant to Lisa, but she turns it down knowing where it came from.

When Lisa turns her focus to astronomy, she immediately tackles the issue of light pollution in Springfield. In desperation, she teams up with Bart to overload the nuclear power station and create a mass blackout. The townspeople gather into an angry mob, but are awed and subdued by the beauty of a meteor shower.

The first-and-only Simpsons feature sees Lisa continue her environmental crusade, convincing the people of Springfield to clean their heavily polluted lake. Naturally, things go awry for the family after Homer dumps an entire silo of pig waste in the lake and the Environmental Protection Agency (EPA) traps the town in a giant dome.