The Red Death rampages, Prospero leads retreat to his secluded castle/abbey, and 1,000 knights and ladies hole up to party like it's 1399.
Prospero decides to throw a masquerade ball, and the narrator decides to tell us all about it. The party's a roaring success. The creepy clock strikes midnight and a new guest appears, looking strangely like a victim of the Red Death.
Prospero orders the guest seized but no one obeys, and the guest stalks slowly to the black room. Prospero draws his dagger, charges after him, and dies. Everyone else pounces on the guest, only to discover it's the Red Death. Then everybody dies and everything goes dark.