THE PLOT

A terrifying disease called the Red Death ravages the dominion of Prince Prospero. So lethal is it that it kills within a half-hour after the onset of its symptoms: sharp pain, dizziness, and bleeding from the pores. 

.......However, the prince is safe and happy in an abbey to which he has withdrawn with a thousand knights and ladies selected from his court. The abbey, which resembles a great castle, is surrounded by a sturdy wall. Its iron gate has been welded shut, making it impossible for anyone to enter or leave.
.......Inside, the prince has stocked food and drink aplenty and maintains companies of musicians, dancers, and clowns for entertainment. 
 After about six months, while the disease was taking its toll outside, the prince held a masked ball in a maze-like suite of seven rooms specially decorated according to a theme color. One room was blue; the second, purple; the third, green; the fourth, orange; the fifth, white; and the sixth, violet. A stained-glass window in the wall between each of these rooms and the outside corridor matched the color of the room. The seventh room was hung with tapestries of black velvet. However, here the stained-glass between the room and the corridor was scarlet instead of black.

There were no candles to light any of the rooms. Rather, illumination was provided by a brazier of fire set on a tripod in the corridor outside each of the stained-glass windows. Thus, shimmering blue light, mimicking the movement of the leaping flames, illuminated the first room, shimmering purple light illuminated the second room, and so on. Into the seventh room, the black one with the scarlet window, the fire projected blood-red light that was ghastly to behold. The masqueraders were reluctant to enter this room. Adding to the foreboding atmosphere of the room was an ebony pendulum clock that tolled the hour with a deep chime that echoed through the winding hallways and unnerved all the guests. 
.......Nevertheless, the party is a smashing success overall, with the guests–outfitted in every manner of odd, alluring, and grotesque costumes–enjoying themselves immensely. But no one enters the seventh room. Instead, everyone congregates in the other rooms. 
.......After the ebony clock strikes twelve, the revelers in the blue room, where the prince is mingling with his friends, notice a new masquerader among them. They express surprise, utter whispers, and finally recoil in terror and disgust. And no wonder. This masquerader, tall in and thin, is outfitted as a corpse in a grave. His mask is as stiff and fearsome as a dead man’s face. Daubs of red on his costume make it clear that he has come in the guise of the Red Death. Prince Prospero reacts with a shudder signifying fear or disgust. Then he becomes angry. He asks, “Who dares insult us with this blasphemous mockery?”
.......Prospero orders the unmasking of the intruder and declares that he will be hanged in the morning from the fortress’s battlements.
But no one undertakes the task. The intruder then moves from room to room. Prospero withdraws a dagger and chases him. In the black room, the intruder turns and faces Prospero. There is a cry. The dagger falls to the sable carpet. Then Prospero falls. Finding courage, Prospero’s friends then attack the intruder. To their horror, they discover that there is nothing inside the costume or behind the mask. 
.......Poe ends the story by revealing the identity of the intruder:

And now was acknowledged the presence of the Red Death. He had come like a thief in the night. And one by one dropped the revellers in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall. And the life of the ebony clock went out with that of the last of the gay. And the flames of the tripods expired. And Darkness and Decay and the Red Death held illimitable dominion over all.