Plot

Green Day is performing on Lake Springfield, where they fail to engage the audience in talking about the environment. They are killed when the pollution in the lake eats away at their barge, causing it to sink. At the funeral, Grampa has a religious vision, in which he predicts impending doom upon the town, but only Marge pays attention to it. Lisa and Colin, an Irish boy, hold a seminar entitled "An Irritating Truth", where they convince the town to clean up the lake by putting the same polluted water in their glasses. Meanwhile, Homer dares Bart to skateboard nude to Krusty Burger and back, but Bart is caught by Chief Wiggum. Ned Flanders comforts Bart after being humiliated, since Homer lets Bart get blamed for the incident and then adopts a pig. He keeps the pig's waste in an overflowing silo, which horrifies Marge, who tells Homer to dispose of it safely. While waiting at the dump, Homer decides on a quicker means of disposal and dumps the silo into the lake, polluting it severely and mutating a squirrel. Nearby, Flanders and Bart bond during a hike and discover the squirrel, and it is captured by the EPA. Russ Cargill, head of the EPA, tells President Schwarzenegger that Springfield is extremely polluted and the government must take drastic action. Under the President's orders, the EPA places Springfield in a giant glass dome, cutting off the town's supplies from the outside world.

The police discover Homer's silo in the lake, and an angry mob approaches the Simpsons' home, setting it on fire. The family escapes the dome through a sinkhole in Maggie's sandbox and flees to Alaska. Seeing the dome begin to crack and realizing the people of Springfield will escape, Russ Cargill manipulates the President into ordering the town's destruction. In Alaska, the Simpsons see a television advertisement in which Tom Hanks promotes a new Grand Canyon, and Marge realizes that they are planning to bomb Springfield. Marge and the kids decide to save the town, but Homer refuses to help the people that tried to kill him. His family abandons him and returns to rescue Springfield. After a visit with a mysterious Inuit shaman, Homer has a vision and reaches an epiphany: to save himself, his family, and Springfield and he returns. Meanwhile, Marge, Lisa, Maggie, and Bart are captured by Cargill and returned to Springfield. Cargill tells the people of Springfield that the town will be destroyed. A helicopter arrives and lowers a bomb. Meanwhile, Homer climbs the outside of the dome and descends knocking the escaping townspeople and bomb off the rope. However, he notices a motorcycle, grabs Bart, and cycles up the side of the dome. Bart throws the bomb through the hole, detonating it outside and shattering the dome. The town praises Homer, who rides off with Marge on the motorbike into the sunset. The film ends with everyone restoring Springfield, including the Simpsons' home, back to the way things were.