There is a Pestilence called the Red Death that kills in 30 mins by blood coming out of the pores. To avoid death, Prince Prospero locks up him and 1,000 of his friends in the keep of his castle.
Prince Prospero holds a masquerade to get attention away from the Red Death, but people are still aware of the pestilence because there is a clock in the red and black room that chimes every 30 mins reminding that the Red Death is killing.
Masked intruder shows up at the masquerade. The intruder has the Red Death. Prince Prospero chases the intruder through the rooms and dies in the 7th room, in the red and black room.
One by one everyone dies at the masquerade from the Red Death because of the infected intruder.
Everyone at the party is dead from the Red Death. The moral of the story is you can't escape death, not even if you lock yourself and your friends in your keep to get away from death because death always finds it's way.
In the Masque of the Red Death, there is a lot of symbolism. The clock represents the lifespan. The masquerade represents a illusion to escape from reality. The intruder represents death and that you can't escape from it.