Enotes has a great study guide for this story.  I have attached the link below.  Here's a general summary to get you started:

Characters:  The narrator, his childhood friend Roderick Usher, and Roderick's twin sister Madeleine.

The Plot:  The narrator travels the Usher mansion after receiving a letter from Roderick.  Roderick complains of having a nervous condition and needing the help of his friend.  It is clear that both the Ushers have some illness.  Madeleine is glimpsed but never approaches othes.  Roderick is nervous and paranoid.  While visiting, Madeleine dies.  Because her condition was "odd", Roderick plans her immediate burial in the family catacombs to prevent having her body examined by doctors.  However, it turns out she wasn't dead.  During a stormy night, she breaks out of her tomb and comes to embrace her brother, who dies with her on the drawing room floor.  The narrator flees the house, which is hit by ligtening, cracking open and crumbling.

Theme:  One of the themes is madness and insanity.  Poe's descriptions point to the the increasing hysteria of the household.  The narrator himself is affected.  Roderick's paranoia and twitchy manner begin to affect his mind and leave us to wonder how much being described is accurate. 

Symbolism:  The crumbling mansion is a symbol for the crumbling state of Roderick's mind.