Eccentric and psychic New York City private investigator Val Cassidy and his brilliant and loyal partner Mabel Deuce, solve the mystery of murderous scionites, creatures looking like humans but composed of a lattice network overlaying human DNA.
Recovering from an assassination attempt in a local hospital, Val develops his own therapy by honing skills at throwing lethal darts. A hospital full of corpses results from the attempt, but the intrepid—and unscathed—detective departs for his office in the warehouse district where he and Mabel piece tougher evidence that a series of murders in upstate New York have the kind of supernatural aura they like to pursue.
As they examine the circumstances of the victims’ murders and work closely with two eccentric female doctors (Asplundh and Sundt) who share their quest, they become convinced that the murderer is now stalking them.
They return to their office to prepare for the onslaught of the scionites. In fact, only one (apparently female) creature arrives, and she is a formidable opponent, finally laid low by Cassidy and Deuce working as a team and using their psychic powers as well as Cassidy’s dart throwing ability.
Forensics determine that the scionite communicates with others of her alien species. Cassidy and Deuce must hazard all to cut off the controller of all scionites and end the menace forever. Their prey is a surprise, but they accomplish their mission.
Have they completely eliminated the threat? Val’s inerrant intuition suggests not. Yet he returns to other matters, scouring the newspapers for hints of dark forces lurking in the shadows.