TOH I:
Opening scene:
The episode starts off with Marge giving us a warning speech about how scary the episode will be and she asks the viewer to put their children to bed early instead of sending angry letters the next day. She also mentions that „nothing seems to bother“ her children.
First segment:
We see bart and lisa sitting in the family‘s treehouse telling eachother horror stories. (hence treehouse of horror lol) Lisa finishes her story and bart complains about her stories being boring and asks for the flashlight. He then starts telling the story of „the bad dream house“.
In the story, the simpsons family moves into a new house, a very obviously haunted house. Marge wants to leave but Homer wants to stay because the house was cheap. They agree to stay the night to see everyone changed their mind. During the night, the house convinces the family to kill eachother. Everybody (but marge) takes a knive and they start chasing eachother in circles. Marge stops them and they confront the house. Lisa asks it if doesnt even want to try getting to know them. The house says it‘ll think about it and the simpson family walks out to give it space. Then they see the house destroying itself. Lisa says how the house would rather destroy itself than live with them.
Second segment:
Bart says that the first story was just a warmup to the second one. He starts telling the story of „hungry are the damned“. It starts with the entire family out in their garden grilling burgers when suddenly lisa sees a UFO flying above them. The UFO picks the family up one by one and Kang and Kodos welcome the simpsons onto their ship and offer them a delicious dinner. They eat up and make a suspicious remark about marge looking delicious. They have a few casual conversations with the family and offer them another round of food. At one point, Lisa becomes suspicious and runs off to the kitchen and finds a cook book that says „how to cook humans“. Lisa runs to the family and they all confront the aliens. It all ends up being just a misunderstanding and the simpsons get kicked off of the ship and brought back to their backyard.
Third segment:
Lisa is reading a old poetry book and bart asks what shes reading. Lisa offers to read hime one of the scary poems and bart refuses but she reads it anyways. She reads the one about the raven. It starts off with homer sitting infront of a fireplace where he hears a knock coming from the door. After gathering up the courage, he first asks whose there and went to take a look but the hallway was completely empty. He closed the door only to hear the knocking once again. Thinking the knocking may have come from the window, he opened it only for a raven to fly inside. When he asks the raven what its name is, it only answers: „nevermore“. Homer yells at the raven to leave but it refuses to leave and only keeps repeating: „nevermore“. Homer then attacks it and chases the raven through the room. After Homer runs into a bookshelf, the raven flies back to the spot it sat at before and laughs at homer.
Outro:
The episode ends with Lisa, Bart and Maggie leaving the treehouse. Homer, who was eavesdropping the whole time cant sleep now and begs marge to keep the lights on. Marge refuses and the last thing we see is homer saying: „I hate Halloween“
Not canon:
The last sequence is a poem told by lisa and they just used homer to portray it but it isn‘t something that actually happened.
Unsure:
The first segment could very well be inspired by actual events (like the second segment likely is) but whether it is or isn‘t is unsure.
Canon:
Believe it or not, there seems to be some type of canon behind the second segment. In the episode „so its come to this“ marge recalls being picked up by aliens, which means that bart‘s alien story may have had some type of truth or actual story behind it.
Connection to gravity falls:
In gravity falls there are also aliens mentioned and a haunted mansion (the northwest mansion). I couldnt find a big similarity between the mansion in the simpsons and the one in gravity falls so i couldnt really build a theory up on that.