The script was written by Charles Spaak and Duvivier, from the 1937 novel of the same name by John Dickson Carr.

[1] Historian Michel Boissard (Walter Giller), is invited with his wife Marie (Édith Scob), a descendant of Madame de Brinvilliers, the notorious poisoner, to the château of Mathias Desgrez (Frédéric Duvallès).

Mathias is adept in the occult arts, which he practises with his friend Dr. Hermann (Antoine Balpêtré).

One night, a little time later, Mathias dies having received a visit from a mysterious woman bringing him medications, and is seen by a servant.

Mathias is buried and then his body disappears just when the analysis proves that he has undoubtedly been poisoned.