In "G-G-Ghost D-D-Dad", Homer chokes to death on a piece of broccoli and his spirit must perform a good deed in order to enter heaven. "Scary Tales Can Come True" reveals the dark side of fairy tale characters and in "Night of the Dolphin" - The world is overtaken by dolphins after Lisa frees their leader from captivity.
The titles are a spoof of The Munsters opening sequence, presented in black-and-white and with the Simpsons theme done in the style of The Munsterstheme.
Homer is Herman Munster, Marge is Lily Munster, Lisa is Marilyn Munster (holding a book on copyright law), Bart Simpson is Eddie Munster, and Abe is Grandpa Munster. At the front of their mansion, an angry mob of townspeople confront the Simpsons. They stab Marge and Grampa, set Homer on fire, and set a leghold trap on Bart. They run off, leaving an unharmed (and non-monstrous) Lisa to sidle away.
At breakfast, Homer reads his horoscope, where it states that he will die that day, and someone will compliment him at work. Marge's horoscope says that her husband will die. Homer dismisses it and leaves for work. On his way to work, he narrowly escapes death several times. When he gets to the power plant Lenny compliments him... about the rattlesnake on his arm.
Since he is still alive at the end of the day (despite multiple injuries and close calls) he scoffs at the horoscope, but is finally killed after eating some broccoli, which Dr. Hibbert informs Marge is "the deadliest plant on Earth - why, it tries to warn you itself with its TERRIBLE taste!".
When Homer reaches Heaven, St. Peter informs him that he hasn't done a good deed in his life, and in order to enter Heaven he must return to Earth as a ghost and do a good deed within 24 hours.
Over the next 24 hours he tries to do good deeds, but succeeds only in getting a noogie from Nelson and accidentally dropping Agnes Skinner to her death (Homer's claim that he's pretty sure she was going to be the next Hitler doesn't pass muster). With only one minute left, he grabs a baby from a runaway pram which then rolls into traffic and explodes.
Confident that he will now get into Heaven, he returns to St. Peter, who apologizes as he had not been looking during Homer's good deed. Thus Homer is sent to Hell, where Satan gives him noogies and asks him to stop yelling because he'll wake up John Wayne, who it turns out has already gotten up for his day in Hell.
The Simpsons are peasants living in a pumpkin cottage, with Homer as the village oaf. When Homer is fired and they cannot afford food, he vows that no child will go hungry in his home. He then tosses out Bart and Lisa into the nearby woods, where they find a former son and daughter pair (who were also tossed out by Homer), now skeletons. Lisa, using her book of Grimms' Fairy Tales, guides them through the forest's many dangers, including a troll hiding under a bridge and the Three Bears.
Meanwhile, Marge admonishes Homer for throwing out the kids, and orders him to get them back. While Homer is looking for his children he finds Rapunzel's tower.
Rapunzel asks Homer to rescue her and lowers her hair so that he can climb up to her window. But Homer is too heavy, and he is only able to climb part of the way up over Rapunzel's pained cries before his weight separates Rapunzel's hair (and possibly scalp) from her head, causing Homer to plummet to the ground. Homer pushes the pile of hair behind a bush with his foot and runs away.
The kids go to a gingerbread house where a witch invites them inside. Soon Lisa is in chains and sweeping, and Bart is being fattened up to be eaten.
Before the witch can cook them, they distract her by saying she has no one to look good for, and when she claims her date (George Cauldron) is coming by soon for a date, Homer bursts in. The witch zaps him repeatedly, making him part-chicken and part-fish among other things. She tries to stuff him in the oven, but Homer overpowers her and shoves her in, locking the door. As she dies, Homer reverts to normal, and the family's surprised when a very handsome man named George Cauldron shows up for a date with "Suzanne".
Later, the family is eating supper. Marge says that although they are poor, they will never go hungry again thanks to Homer, who can now lay eggs with his chicken lower-body-half.
Lisa goes to Marine World, a SeaWorld-like park where she takes pity on a dolphin called Snorky. Lisa sets Snorky free (in a homage to Free Willy), but unknown to her, he is the king of the dolphins, and resents humans for forcing him to do tricks. He plots to take over the world. The dolphins begin their war by attacking Springfield, killing Lenny in the ocean (Chief Wiggum sees Lenny's obviously dolphin-caused wounds and blames rowdy teens for his death) Snorky strikes back and kills the Sea Captain and start marching towards the town on their caudal fins.
The dolphins go all around Springfield killing various characters like Squeaky-voiced Teen,Carl Carlson, Kent Brockman and Raphael, and Snorky interrupts a town meeting and kills Groundskeeper Willie by breaking through the windows and then right through him, he takes the stage and tells Springfield that dolphins used to live on land, but were banished to the ocean by humans. As dolphin king, he then banishes all humans to the ocean.
After a speech by Homer about mankind's many achievements, the humans decide not to submit to the dolphins' will without a fight. As the humans are walking out of the hall, Lisa sees a small dolphin with a set of bottle rings around its face. She takes it off and it bites her, making her yell, "Son of a...!" - she is cut off by Homer, and a battle ensues between the dolphins and the humans, lots of people are injured, Abraham Simpson is eaten by one of the dolphins, and a few citizens are killed. In the end the dolphins win the fight, and we see the Simpsons and other residents of Springfield floating in the ocean. Lisa regrets that her actions "doomed mankind", while Marge tells her that she is exaggerating and that they will just have to adjust to living in the ocean. Then Krusty's corpse floats over to her, disgusting her and making her nudge it away. Krusty's body floats over to join many other corpses; an overhead shot then shows the floating bodies are lined up to spell "THE END?".
After the last story, the two aliens, Kang and Kodos, complain that they have been left out of the Halloween special. Suddenly, they receive a phone call from Old Navy, asking if the Rigellians can do a commercial, which they accept.
The episode received generally positive reviews. Den of Geek said "it is probably one of the funniest Halloween episodes of the show's history". The episode got a 4.02 rating out of 5 ranking 69 out of 373 episodes. Matt Groening called the third segment his personal favorite scenes from all the series.
The episode got a 8.4 reader rating from TV.com. Colin Jacobson of DVD Movie Guide gave the episode a positive review saying "Even in the weakest Simpsons seasons, you can count on the Halloween episodes to deliver good amusement. Or at least pretty decent comedy, as evidenced by the up and down “Treehouse XI”. None of the segments excel, but none of them flop either, so they keep us entertained. It’s really hard to fault a mainstream network TV series that references glory holes, so “Treehouse XI” gets a positive appraisal despite a few missteps."