Due to a shipping mix-up, The Simpsons get a swimming pool installed and become the talk of Springfield, or at least until its intended recipient Ned Flanders finds out and tries to get it removed. Then it becomes war.
One hot Summer day, the beach is closed to the public so that Mr. Burns and Smithers can play Battleship (with actual battleships), and The Simpsons are forced to stay at home and try to beat the heat. When loud sounds are heard from the backyard, Homer heads out to find that a construction crew is busy installing a swimming pool for whom they believe is Ned Flanders. Naturally, Homer goes along with it.
It isn't long before the pool becomes all of Springfield's favored Summer hang-out, but the pool party comes to an end when Ned and his sons return from their own vacation and finds that the pool he had ordered as a Summer surprise for Rod and Todd got put into the wrong backyard. The construction crew is hired to seal up the pool, but the pool-goers refuse to clear out and sides with Homer in order to defend their watering hole.
Fortunatley, Herman is on hand to provide military supplies for what proves to be a lengthy occupation. Bart is all for this encampment (as it means it'll be that much longer to avoid school) and offers the Springfield youth to assist, while Milhouse volunteers to tunnel behind enemy lines. With barbed-wire on the perimeter, the Springfield citizens patrol to defend their pool in spite of the pool company's harsh retalitory tactics, such as horrid music and cutting the power.
As the pool company sends in bulldozers to take down the camp by force, Milhouse accidentally knocks down a support beam while digging and causes the pool and much of the backyard to completely collapse. The pool company laughs at Homer's karmic retribution and leaves, and all the citizens vanish now that there's no pool. Poor Homer is left to fix the backyard all by himself while Marge and the kids stay with Patty and Selma's.