When the first threats appear they are assumed to be a hoax. But when a hundred and twenty people are threatened with death, and only a few live because they pay the ransom, while the others die from natural causes or accidents then it is no hoax.
Lucifer, a paranoid young man, believes that he is Satan and has a remarkable psychic ability to predict death by natural causes. Lucifer's ability is used by Seff's gang to extract ransom from their rich victims. Seff selects the victims and warns them of their impending death, any who does not pay the ransom seemingly dies from natural causes, helped on their way by Seff's executioner, Jack Wish.
By what means are the victims being killed? How is the ransom money being collected without trace from under the sea?
Modesty's friend, Steve Collier, the renowned psychic researcher, is kidnapped to help Seff and he accidentally blows Modesty's cover as she investigates this mystery. Taken prisoner Modesty and her faithful lieutenant, Willie Garvin, must escape Seff's island stronghold.