The story revolves around the efforts of a scientist, Dr. Lowell (a pseudonym he uses because of his fear that the incredible and irrational events he relates will discredit him as a scientist), and a gangster, Julian Ricori, as they first uncover the existence of the powerful witch of the title and then try, unsuccessfully, to find an effective countermeasure to her lethal magic. They track her to her lair, where they confront and finally kill her.
In the beginning, Dr. Lowell relates that Ricori has sought Lowell’s services on behalf of the gangster’s lieutenant, Thomas Peters, who suffers from a severe shock that has destroyed his nervous stability. His eyes are wide open, his posture is rigid, and, although neither unconscious nor dead, he is unresponsive to treatment. Lowell struggles to find a medical solution to the problem but cannot. He slowly (if grudgingly) concludes that Peters suffers from some sort of undiscoverable evil “presence.” Peters is under the influence of Madame Man-dilip, whom Lowell has not yet met, though he infers her power to manipulate her victims psychically and from a great distance.
Peters dies while Lowell helplessly attends him. Lowell and Ricori discover that other persons have met similar fates. The death of one in particular, Nurse Walters, provides them with a diary that details her visits to Madame Mandilip’s doll shop. It contains both one of the greatest gothic descriptions of an occult event in American fantasy fiction and enough information to allow them to identify Madame Mandilip as the source of the necromancy that they have decided is the root of the eerie and lethal events that they have been investigating.
Later, Lowell and Ricori abduct the witch’s assistant, the pitiful and frightened Lascha, and learn enough to reach the conclusion that Madame Mandilip is an ancient witch of great and malign power and that she is capable of assuming many physical forms. They then make their plans to confront her in her hellish doll shop and engage her in a battle to the death, an encounter that is among the best of its kind in American fantasy fiction. Although they finally conquer supernatural evil, they do not escape unscathed, either physically or psychologically.