Prince Prosper isolates himself along with some of his friends. They isolate themselves in an Abbey away from the town where the disease is spreading. The Prince throws a party, with all his friends to get his close ones away from the killer disease.
A disease called the Red Death is spreading throughout the town killing everyone it comes in contact with. The Prince isolated himself with his friends in the Abbey so they wouldn't catch the disease. The Abbey was Castellated so nobody was around the area besides the Prince and his guest.
A new guest arrives at the party they are throwing. Nobody recognizes who the arriver is. They "stalk" him throughout the party to see who he is and what he is doing. The Prince follows him through seven different rooms and finally confronts him.
The Prince draws a spear, ready to kill him. As he reaches, ready to kill him, the Prince dies. Along with that, the man in the cloak dissapeared. All along, the thing in the cloak wasn't a person, maybe the Red Death itself. As the thin air was gone, the disease began to spread.
Many others went to go see what was going on. As soon as they reached the spot where the Prince died, they all soon began to die too. They were infected with the disease.
In the end it turns out everyone dies. the man in the coat was indeed not a man at all. As everyone died and the disease left the Abbey, the disease lived on and spread to different areas.