Simsons Season 6, Episode 14: Bart's Comet

During detention, Bart accidentally discovers a comet. It is then discovered that the comet is heading straight for Springfield, and a rocket is launched to destroy it. However, it misses and blows up the only bridge out of town, dooming everyone in Springfield. The Simpsons and the Flanders share the bomb shelter that Ned had built, to hopefully protect themselves from the comet. Soon, everyone in town joins them in the shelter, but the door cannot be closed unless somebody leaves.