"Bart of Darkness" is the first episode of Season 6 (originally produced as the Season 5 finale).
In a parody of the Alfred Hitchcock classic Rear Window, after breaking his leg, Bart spends the first several weeks of summer at his bedroom window, spying on the neighbors with a telescope. Bart finds that staying in his room is boring, that is before he thinks of witnessing Ned Flanders murdering Maude. Did such a thing really happen? Will Bart uncover the truth about this?
At the start of a very hot summer vacation, the Simpsons' fridge breaks after a failed attempt to beat the heat. Fortunately, Otto comes to their salvation with the "Pool Mobile". Bart and Lisa and the rest of the neighborhood kids enjoy the pool time until Otto tells them that their time is up and that they don't have enough money in the budget to come back. Upon hearing this, Bart and Lisa convince Homer, rather easily, to buy them a pool. After mistakenly building a barn on the first try, the Simpsons build a proper pool and suddenly Bart and Lisa become the most popular kids in the neighborhood.
All the kids at Springfield Elementary, as well as kids they don't know (and who also don't know them), arrive at the Simpsons house every day to swim. However, when Bart is dared to jump from his treehouse into the pool, he gets distracted by Nelson ("Your epidermis is showing!") and falls and breaks his left leg. Bart is forced to stay indoors with a cast on his leg while Lisa enjoys her newfound popularity with the other kids swimming in their pool. Even though she knows she is only being used for the pool, she ignores that thought believing she really has found some friends. Marge and Lisa soon take notice that Bart is becoming isolated and weird. To suppress Bart's sudden paranoia, Lisa gives Bart her telescope for something to do. He first comes to the conclusion that the universe is boring and instead uses the telescope to spy on Springfield residents. He finds nothing even remotely interesting to spy on, until he hears a female scream and observes Ned Flanders burying an object in his backyard.
After hearing a conversation between Ned, Rod, and Todd, Bart thinks that Ned will kill them as well. Lisa becomes unpopular again when Martin sets up a better pool in his backyard. Bart convinces Lisa to sneak into Flanders' house and snoop around for evidence. However, Bart sees Ned coming into his house with an axe. Certain that Ned intends to kill Lisa, Bart attempts to call 911 only to receive an automated response from a recording. Bart plans to rescue Lisa himself and struggles to get downstairs and out the door, encountering many setbacks on the way to Ned's house. Meanwhile, Lisa realizes Ned has returned home and sees him carrying an axe. She hides in the attic and Ned follows her. Bart arrives just in time to see Ned put the axe on a mount secured the wall. He is dumbfounded when he sees Bart, who confronts him, demanding to know why he killed Maude. Ned is suprised to hear Bart say that he murdered Maude and passes out.
The police arrive to the scene with both families, including Maude. Maude explains that she was actually at Bible camp for a week, learning how to be more judgmental. When Bart asks Ned about the "murder" that he saw, Ned breaks down and says that he accidentally overwatered Maude's favorite ficus plant, panicked, and then buried the remains; he had planned on replacing it before Maude came home. Bart then asks about the woman that he heard scream, and Ned says that he can't explain that. When Lou digs up Maude's dead ficus, Ned lets out a shrill-like scream. Bart then concludes that everything is normal, and Homer gets sarcastic for a moment.
Meanwhile at Martin's house, his pool becomes too overfilled and bursts. Everyone leaves and Martin becomes unpopular again like Lisa. Nelson snatches Martin's bathing suit off, and Martin stares naked into the sunset as the credits roll.