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 Homer refuses to give it up despite Lisa suggesting he give in cause of a previous incident with the Gideons going so far as to blow up Homer's car to get their Bibles back. Marge also is upset at Homer cause he refuses to let anybody leave the pool using them as "hostages" referring to Homer having a "jerkass moment". The crew mocks Homer telling him to listen to his wife or no dessert but Marge takes offense to this.
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. In spite of the pool company's harsh retalitory tactics, such as horrid music and cutting the power, Homer refuses to give in despite the townspeople ready to give in.
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. In the meantime, the citizens are happy to be free of Homer's tyranny when released but Homer refuses to quit, while the crew actually decides to pull back thinking that killing a family over a pool is stupid. 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.