Suffering under a great plague, Salem's leading witch, Mary Sibley, faces both foreign and domestic challenges to her authority.
Sebastian wants to run away with Mary, but she feels a duty to Salem. Cotton is happy about his offspring, however he has no idea that witchcraft is actually responsible for the pregnancy.
The witches bring Satan to earth and make Salem his capital. But instead of freeing the New World from the murderous Puritans, he pursues his own plans. He wants to bring slavery and death upon humanity.
Mary begins the second act of the great ritual, at the same time as Alden returns to Salem. Meanwhile, Dr. Wainwright searches for clues to cure the plague, using worrying methods.
Mary's penance for her betrayal takes a deadly turn while Cotton is forced to perform an exorcism of his own.