This classic short story by Edgar Allen Poe is not actually that long, so it is well worth reading for yourself so you can gain a better idea of what precisely happens. However, just to get you started, I will outline some of the major events that occur to hopefully support you as you return to this story and read it.
We are introduced to a supposedly medieval setting which has been struck by a plague called the Red Death. In an attempt to escape this terrifying disease, which is noted for the way in which victims expire blood as they die, Prince Prospero and numerous courtiers have ensconsed themselves in a castle with all the luxuries they need, with nobody allowed to enter or exit the place. In this castle they dedicate themselves to luxury and decadence. One night in particular, an elaborate masked ball is held where everyone is dressed in disguises. The setting is quite spectacular, as there are seven rooms running from East to West, each with a different colour. The last room is in black and has a clock that, every hour, chimes. Few people go into this room for it is so terrifying, and when the clock chimes, people stop their merriment for a moment. However, people become aware of a guest who is dressed as a corpse who has been killed by the Red Death. Prospero, so angered by this, pursues the guest through the rooms until, reaching the seventh, black, room, the guest turns and confronts Prospero who dies immediately. The presence of the Red Death is acknowledged, and all of the guests die, one by one. It is not so easy to escape death after all, it appears.