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In 12th-century Italy Prospero, a Satan-worshiping prince, jails two peasants, Gino and Ludovico, for defying his authority to tax citizens.  Francesca, Ludovico's daughter and Gino's fiancée, pleads for their lives, and the intrigued Prospero agrees to spare whichever one she chooses.

Later, the prince sees evidence of the Red Death plague in the village and orders all houses in the infected area burned.  The disease obliges Prospero to retreat to his castle, and he forces Francesca to accompany him, intending to dress her in courtly attire and have her watch him indulge in sadistic pleasures as part of her instruction in diabolism.  Juliana, the prince's mistress, is jealous of Francesca but aids her attempt to help Gino and Ludovico escape.

The plot is foiled, however, and at one of the events preceding Prospero's annual masked ball, the prince orders the men to cut themselves with five knives, one of which is poisoned.  Ludovico is impaled on Prospero's sword when he attempts to kill the prince with one of the weapons, and Gino is banished to the burning village, promising to return.

Juliana, meanwhile, sacrifices herself to Satan while Prospero watches unperturbed.  On his way back to town, Gino meets a strange figure in red, who takes him back to the castle and instructs him to wait outside for Francesca.  As the mysterious intruder enters the ball, Prospero and his guests die of the Red Death, but Gino and Francesca are permitted to survive.