The narrator is invited to House of Usher by Rodrick Usher because he is in need of help and he thinks that his friend from his childhood, the narrator, can help him.
"At the request of Usher, I personally aided him in the arrangements for the temporary entombment." (23)
The narrator and Usher meet. Usher tells the narrator that he's is mentally sick and it is taking a toll on him physically as well.
"We have put her living in the tomb," Usher said, "Madman! I tell you that she now stands without the door!"
The narrator is told that Usher has a twin sister named Madeline and she is dying because she is cataleptic. "The disease of the lady Madeline had long baffled the skill of her physicians. A settled apathy,... transient affections of a partially cataleptical character." (19)
They find Madeline dead so Usher asks the narrator to assist him in putting Madeline in a coffin to rest.
Usher realizes that they have put Madeline who is alive in a coffin. She kills him.
Because Usher and Madeline are the last of the Usher family, the house collapsed because Madeline and Usher died.