Every time the clock strikes an hour, everyone stops to listen and reflect.
Prince Prospero does not seem to care much about his people. Rather than take care of them during a time of trouble, he takes care of himself. His kingdom is besieged by a terrible plague called the Red Death, which kills you within the hour. He decides to take about a thousand of his closest, richest friends and hide away inside “one of his castellated abbeys.”
Prospero and his people have walled themselves in well, but Prospero decides to decorate the castle with seven voluptuous rooms. These are decorated each in their own garish color scheme, with the last “shrouded in black velvet tapestries” with scarlet window panes and a “gigantic clock of ebony.”
The room, and particularly the clock, clearly creeps everyone out. They avoid it. When the clock strikes, something peculiar happens. The people seem to ponder their mortality. Musicians stop playing to listen, and the dancers stop dancing. Everyone stops and listens to the clock.
[There] was a brief disconcert of the whole gay company; and, while the chimes of the clock yet rang, it was observed that the giddiest grew pale, and the more aged and sedate passed their hands over their brows as if in confused reverie or meditation.
The clock seems to remind them that while they may be safe and sound within, there is death and pestilence without. How much longer can they remain in their cocoon? How safe are they really? Has Prospero’s charade really worked? Have they really cheated Death?
The answer comes in the fifth or sixth month of their seclusion at midnight, when an uninvited guest arrives at the party. The “mummer” seems to be in the costume of the Red Death. The guests deem this in bad taste, but Prospero is incensed with anger. He tries to attack the "man," running through all seven rooms and stopping in the one with the clock.
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.
As the clock strikes its last, they are all dead—including Prospero. The clock, of course, is symbolic. It is death. The people were reflecting on their own mortality, seeming to know every time the spooky clock struck that they were going to die soon.
Death was not invited to the party, but he arrived anyway. Prospero’s fantasy that he could shut himself away and create a world where he could escape death was unrealistic. One way to look at the seven rooms is as the stages of a person’s life. The last one is death. This is why Prospero runs through them, and also why they have windows for the sun to rise and set through, with the last one having red panes. Prospero believed that he could cheat death even in creating this elaborate mimicry of the passage of life, but he was wrong.