In Victorian England, a series of gruesome deaths has begun to terrorize the countryside. The victims, discovered with inexplicable marks on their bodies, appear to have been drained of blood. Enter Inspector Quennell, a determined detective from Scotland Yard tasked with solving these mysterious killings. As he delves deeper into the case, Quennell encounters a range of peculiar characters and clues, including a strange connection to a local entomology professor named Dr. Mallinger. The professor's research into the metamorphosis of insects seems oddly relevant to the nature of the crimes.

As Inspector Quennell's investigation progresses, his path crosses with that of Dr. Mallinger's daughter, Clare. Clare's enigmatic behavior and her father's secretive experiments hint at a sinister secret they are desperate to conceal. With the inspector's suspicions raised, he must navigate a complex web of deceit and danger, pushing him towards a horrifying discovery that challenges both his rational mind and the very nature of scientific understanding. Quennell's methodical approach to the ghastly murders draws him ever closer to a truth that threatens to shake the very foundations of Victorian society.