The stabbing death of a beautiful young woman in a London park at night and the disappearance of her sister; a shocked and heartbroken young suitor; a terrified landlady and her simple husband; a recently unemployed man, the narrator, who is requested to investigate the crime; and the mysterious title character who, as his name suggests, has a devilishness about him in his psychic ability to read minds and to perform other supernatural acts. These are some of the individuals in this inventive, absorbing mystery by the prolific and popular British author, B. L. Farjeon. There are really two mysteries here: the murder and the strange – person? – known as Devlin.