from wikipedia: (of course it's better to watch the episode, for all the subtle gems, imo)
After a Congressional bill to evacuate Springfield is defeated, Homer decides that they should stay in the bomb shelter that Ned Flanders built. Anticipating this, Ned had built it large enough for both families. One hour before Springfield is destroyed, the rest of the townspeople arrive, demanding a place in the bunker. Homer is unable to close the door and someone has to leave. Homer decides that the only thing the "world of the future" will not need is left-handed stores and tells Ned to go. Eventually, Homer feels guilty and leaves as well, followed by the other townspeople and they all converge on a hill to await death. As the comet enters the atmosphere, it burns up in the thick layer of pollution over Springfield, popping Skinner's weather balloon and destroying Ned's bunker on the way. The town decides to burn down the observatory to prevent a similar incident from ever happening again. The Simpsons, however, are more worried at the fact Homer correctly predicted the fate of the comet - that it would burn up and fall to earth as a rock no bigger than a Chihuahua's head (It even lands next to an actual chihuahua in comparison).
