The collection centers on the unsettling case files of Dr. Hesselius, a metaphysical physician who investigates phenomena that blur the line between psychological obsession and supernatural intervention. Through these five distinct narratives, Le Fanu explores the fragility of the human mind when confronted with inexplicable forces. The stories are presented as clinical records, framing the accounts within a pseudo-scientific context that heightens the sense of dread. Each protagonist faces a unique, encroaching terror that threatens their sanity and physical safety, forcing them to confront the limitations of rational explanation.

Readers and critics frequently highlight the collection as a foundational work in the development of the Gothic horror genre. Discussion often centers on Le Fanu's ability to maintain a pervasive sense of unease without relying on excessive gore or jump scares. Many note that the inclusion of 'Carmilla' provides a significant influence on later vampire literature, particularly regarding the depiction of predatory desire. The balance between the clinical, detached tone of the narrator and the visceral terror experienced by the characters remains a primary point of interest for scholars of Victorian literature.