A volcanic eruption occurs in Springfield, and residents try to figure out what to do.
Homer falls asleep while working in the Nuclear Plant when a meltdown occurs. Carl has to deploy the emergency foam, and Homer is told by Smithers to go to Mr. Burns's office. When Homer goes to Burns's office, he finds Smithers crying and Smithers tells Homer that Burns is working at another power plant. Eventually, Smithers encounters Burns at a geothermal energy plant, tapped into Springfield Volcano (which has been dormant), and Burns explains that he needed a backup plant for his financial security. Smithers starts pressing buttons on the control panel, causing the plant to go into overdrive, and a few hours later, the volcano erupts.
Everyone begins to panic and leave town, but the erupting lava is blocking exit routes. Eventually, Mayor Quimby pulls out the town charter, which states that "the most innocent person in town must be thrown into the volcano to appease the angry gods!", and Quimby suggests finding the dumbest person in town to sacrifice. A competition is held to determine who is the dumbest person in Springfield, and the competition is narrowed down to eleven people:
Blonde woman with mole who slept with Mayor Quimby (name unknown)
The first test is getting a kite down from a tree. Homer crashes head first into the tree. Ralph thinks he is a beaver and gnaws on it. Cletus makes a human slingshot out of his children, as he says they're rubbery from a "roadkill and moonshine diet;" they launch Cletus into the tree (a la Angry Birds) and like Homer, crashes head first. The next test is a simple quiz, with Ralph and Cletus giving nonsense answers to Krusty's questions, and Homer is still trying to get the kite down while "choking" the tree (as if choking Bart). The results are in, and there is a three-way tie between Cletus, Homer, and Ralph for the dumbest person in Springfield, which means all three will be sacrificed. Chief Wiggum has a plan to swap out Ralph with a fat Ralph doppelganger.
Lisa thinks there is another way to fix the volcanic eruption without the need for human sacrifice. She, along with Homer, Cletus, and Ralph, go inside the geothermal plant and devise a plan to start pressing buttons. An employee named Hank, who was left behind in the plant, joins in and starts destroying things. The temperature of the lava begins to drop and freezes. A parade is held for Homer, Cletus, and Ralph, with Lisa unhappy that she is not praised for coming up with the idea to save the town. Quimby changes the town charter to read that the dumbest people in town will be consulted first in times of crisis. In the year 2200, Homer comes up with a plan to use a "giant baseball bat" to deflect a giant asteroid away from Springfield, instead of Professor Frink's idea to use giant missiles to destroy the asteroid.