When private investigator Bill Boone is summoned to the secluded California estate of the elderly Forrest Depew to advise on financial matters, he finds himself embroiled in a sinister plot. Depew's greedy relatives are conspiring to have him declared mentally incompetent in order to gain control of his fortune. But the suspicious death of Depew himself suggests an even darker mystery is afoot. Boone must unravel the secrets of The Knoll before the killer strikes again.