In this unnamed year, a plague called the "red death" is sweeeping the country. The "intruder" is actually the "Red Death" Prince Prospero has tried to protect his "thousand friends" from by cloistering them all in his palace.
The story begins: "The “RED DEATH” had long devastated the country. No pestilence had ever been so fatal or so hideous....But Prince Prospero was happy and hauntless and sagacious. When his dominions were half-depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys....All these and security were within. Without was the “Red Death.”
Prospero thinks his loyal followers will be safe within his fortress. But the "intruder" appears among the revelers....nothing can stop death.
The story concludes: "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."