A heatwave in Springfield leads Bart and Lisa to persuade Homer into getting a swimming pool. To avoid months of persistence, he gives in instantly. Word soon spreads that the Simpsons have a pool and every child in the town comes to use the new facility. After being dared, Bart attempts an ambitious dive into the pool from the top of his treehouse, but gets distracted by Nelson, and falls and breaks his leg. This forces him to spend the rest of the summer wearing a cast and, unable to socialize with the other children, he retreats to his bedroom. His isolation slowly makes him more and more irritable and paranoid. Lisa loans Bart her telescope to entertain him. Soon, Bart hears a woman's scream next door, and witnesses Ned Flanders burying something in his backyard and screaming "I'm a murderer!" He becomes convinced that Ned has murdered his wife Maude, and vows to reveal the truth.
Meanwhile, Lisa basks in the glow of her new popularity. However, Martin Prince gets a larger pool, and everyone abandons Lisa, leaving her inside the empty pool with no idea of how to get out. After Lisa apologizes to her brother for neglecting him, Bart asks Lisa to go and look for evidence of the murder while Ned is out of the house. When Ned returns early, Lisa becomes trapped in the Flanders house, eventually being cornered in the attic; Bart painfully makes his way over there by himself, just in time to discover that Ned was actually storing an axe and not trying to threaten Lisa with it. Maude, as it turns out, is actually alive, and had gone to Bible Camp for the weekend, to learn to be more judgemental. The victim of Ned's "murder" was merely her favorite plant, and the woman screaming actually came from Ned himself. Meanwhile, Martin's swimming pool breaks after being full with too many kids, and everyone abandons him, although Nelson steals his bathing suit first. As the episode ends, Martin sadly sings "Summer Wind" to himself as he stands naked in the wreckage of his pool.