While a plague devastates the peasantry, the mad prince Prospero hosts a masquerade ball for the noble class in his castle. When a noblewoman is exiled, her long-lost twin, raised among the lower classes and seeking shelter from the plague, assumes her identity and enter the castle as an imposter. Once inside, she is received into a decadent world of orgies, opium, power schemes, revenge, and decapitations.
