"In a Glass Darkly" is a collection of five short stories in the horror and mystery genres, all presented as cases of a fictional physician whose researches lead him into the dark corners of the supernatural. The tales involve a range of eerie and macabre happenings such as a man being haunted by his doppelgänger, a cursed mirror, a deadly spectral wolf, a case of spontaneous combustion, and the iconic story of a woman being preyed upon by a female vampire. The stories are tied together by the posthumous papers of the occult detective Dr. Martin Hesselius.

The novel revolves around a man who encounters his doppelgänger and becomes obsessed with the striking resemblance between them. This obsession leads him to concoct an elaborate scheme involving identity exchange and insurance fraud. As the protagonist meticulously plans what he believes to be the perfect crime, his narrative becomes increasingly unreliable, revealing his descent into madness. The story unfolds through a complex structure of layered storytelling, blending reality with the protagonist's delusions, and culminates in a darkly ironic twist that challenges the reader's perception of truth and fiction.