What is the name of the person Salvatore may be poisoning with arsenic?
Two impecunious English sisters, Ellen and Agnes Isit (Dulcie Gray and Margaret Johnston), unexpectedly inherit a Neapolitan villa from a deceased uncle and move to Italy to view and sell their property. A local man, Salvatore, has since a boy been employed by the deceased uncle becoming major domo and he now manages the villa and its vineyard. Exploring her late uncles' studio, Ellen uncovers a painting of a nude Salvatore as Bacchus.
Soon Ellen becomes drawn to the carefree life of the locals and the romantic charisma of Salvatore, while the prudish Agnes resists. During the raucous revelry of the grape-treading festival, Agnes succumbs to her suppressed desire. Rushing to the balcony she cries out for Salvatore who drops Ellen and climbs from the grape vat and to her bed.  The pair are quickly married, and husband Salvatore now is master of the estate.
Soon, Ellen becomes aware of a change in Salvatore's behaviour towards Agnes.  Not long after the marriage, Agnes' health begins to deteriorate and Ellen's suspicions are aroused.  She expresses her concerns to a visiting English doctor, Benjamin Dench who is Agnes's former fiance'. Ellen is convinced that Agnes is being poisoned.  She enlists Dench's help in trying to prove that Salvatore is slowly murdering her sister with arsenic.  The villa once belonged to Salvatore's family and he has long been determined to regain ownership. Having poisoned his employer to inherit he had not anticipated the sisters arrival on the scene.
Agnes Isit