The scene is the Mayor’s house in Bolkenhain, a remote village in Silesia; it is the winter of 1662, and the area is still suffering the privations brought on by the Thirty Years’ War. The present Mayor, Silvanus Schuller, is a Dutch merchant who has inexplicably chosen to move there with his glamorous wife, Benigna, the widow of Van Geldern, an immensely wealthy slave trader.

Benigna is haunted by a guilty secret: as a young, beautiful dancer, at age 15, she was the mistress of Johnson, a runaway Negro slave; he forced her to marry Van Geldern, expecting to live off the old man’s wealth. Instead, Benigna found in Van Geldern a man she could love and trust, who treated her gently and accepted her mulatto child as his own. She tried to send Johnson away, with the result that Van Geldern died and Johnson fled, suspected of his murder. The bereft Benigna turned to Schuller, who knew nothing of the story, for solace. When they married, she forced him to move to this isolated place with her daughter Arabella and their three servants: the Catholic Rosa Sacchi, Benigna’s confidante; the dour, half-mad Jansenist Jedediah Potter; and the Huguenot gardener, François Tortebat.

As the opera starts, Potter is showing in Löwel Perl, a wealthy Jewish merchant who is the Schullers’ chief means of communication with the outside world. Effusively greeted by Arabella, Perl is nonetheless aware of something wrong in the house’s atmosphere. Benigna, he learns, is closeted with the Prince Abbot of Hohenwaldau; among the other guests expected for luncheon is the Lutheran Count Hüttenwächter, who is engaged in a bitter land dispute with the Prince Abbot’s monastery. Schuller insists that Benigna has made the house a place of peace and tolerance for all, but Perl’s suspicions are confirmed when strange bickering breaks out among the servants. When Rosa finally gets a moment alone with Perl, she begs for news, explaining that Benigna is beside herself with worry, and Perl tells her the last thing she wants to hear – that Johnson has escaped from prison. Perl produces another blackmailing letter from him, which Rosa takes to pass on to Benigna.

The Count and Countess arrive with the Lutheran Pastor Plebanus Wendt. The men are distracted by the entrance of the serving maid Daga, a subservient creature whose beauty enthralls the lecherous Count. Benigna now enters with the Prince Abbot, and the guests sit down to their meal. The table conversation, though jovial, is rife with tension, heightened by the late arrival of Hadank, the organist of Pastor Wendt’s church. He brings the news that the most dreaded of plagues, the Black Death, has broken out again. Laughter eases the tension, however, and the meal and conversation resume at a giddier pace, Arabella flirting outrageously with the Count.

Suddenly Daga enters in hysterics, claiming she has just seen a man in a black mask climb over the garden wall. The women help the half-fainting girl offstage, while Schuller and François go out to search the grounds. The male guests, left alone, gossip maliciously about Benigna’s first marriage and Van Geldern’s mysterious death. Schuller and Benigna return and offer the guests the unconvincing explanation that the mysterious intruder was either the chimney sweep or a carnival masquer. Rosa rushes in to tell Schuller that a troupe of masquers have somehow gotten into the house and refuse to leave. As he goes out to investigate, a dancer in a death’s-head mask enters the room, whirling around Benigna, who collapses in a dead faint. She is helped offstage by Rosa and Arabella.

The Count, alone with the Abbot, challenges him about the disputed lands, but the conversation is cut short as the Abbot suddenly notices the silent presence of a black man – Benigna’s implacable lover Johnson – who leaves a handprint on the tablecloth and vanishes as quickly as he came. Potter leads the two startled guests to the upstairs gallery before they have time to react. Benigna, who has now read Johnson’s letter, comes with Löwel Perl. In a state of high excitement, she pours out the whole story of her affair with Johnson to him, confessing that her shame over her past has made her unable to consummate her marriage with Schuller; instead she has allowed him to take Daga as his mistress. She is terrified that Schuller will hate her when he learns the truth. Offstage we hear a chorus singing the Dies lrae. Seeing the handprint on the tablecloth, Benigna realizes that Johnson is in the house. As Potter announces Schuller’s return, she dashes out in hysterics.

While dancing and laughter are heard upstairs, Schuller confesses his unhappiness to Perl, who consoles him and leads him up to rejoin his guests. Potter, left alone with Daga, reproaches her bitterly for her relations with Schuller. Cries for help outside reveal that mercenaries have attacked a nearby village; Potter sends Daga out to aid the refugees and hides as the Count enters, hopelessly drunk, filling Pastor Wendt’s ear with scandal about the Abbot. As they lurch out, Potter begins to pray, but is stopped by Johnson, who reminds him that he was an accomplice in Van Geldern’s murder. Potter collapses and dies, his body hidden by the tablecloth, as Johnson hastily disappears.

Hadank, very drunk, now appears courting Rosa, who senses the corpse’s presence and flees in panic, pursued by her would-be lover. Benigna, in terror of Johnson, tries to confess to her husband but cannot bring herself to do so. François announces the arrival of two town councilors, a doctor and a notary, to ratify Schuller’s gift of the disputed lands to the monastery, at Benigna’s request. As Schuller leads them to his study, Benigna confronts Rosa, who admits that Johnson is in the house. As Benigna begins raving, seized with the desire to see him again, the women discover Jedediah’s corpse and flee the room.

Schuller and his guests assemble, drawn by Benigna’s screams. Announcing that Benigna has taken to her bed and demands to see a priest, Rosa leads the Prince Abbot upstairs; Ebbo accuses the Abbot of having seduced Benigna into giving the lands away. Schuller’s angry response is interrupted by the doctor finding Potter’s corpse and pronouncing it dead of the Black Plague, which sends the company into a total panic, as the Dies lrae is once again heard from offstage. In the midst of this, Rosa and the Prince Abbot appear with the news of Benigna’s death. As the others stand in mute terror and consternation, Schuller rushes upstairs and shoots himself.