Springfield is inundated with holiday revelers after the nuclear power plant springs a radiation leak, causing snow to fall all over town. When the tourists drive up Christmas shopping prices, Marge opens up the Simpsons’ house to boarders.
I don’t know off the top of my head why a leak at the plant would blanket the town with snow, and why this would be a tourist draw for America’s “Crud Bucket”. I also don’t know why this snow causing leak would not endanger the Simpsons themselves. I am sure the writers think there is a reasonable answer to these and all my other questions, but I learned a long time ago that there is no point in looking for reasoning behind this particular brand of crazy.
