Lisa convinces the locals to clean up the polluted Lake Springfield after a crowd of fans sink the Green Day concert stage. Meanwhile, Homer saves a pig from being killed in Krusty Burger and adopts it, calling the animal Spider Pig. After two days, Spider Pig fills up a silo with its excrement and not knowing what else to do, Homer dumps the silo in the lake, polluting it. The angry population forces the Simpsons to move to Alaska. Meanwhile, the president is induced by his adviser from the EPA to put a dome over Springfield to hold in the population and destroy the city. When Marge sees the news on television, she tells Homer that they must return to Springfield to save their town and friends, but Homer is not convinced that the people of Springfield deserve their support.