In 1920 Honolulu, a series of human-sacrifice style murders of the elite occur, some of them at heaiu (temples). The hooded killer leaves notes promising more murders to come and are signed: "Cult of Ku". Grant Kingsley, son of a prominent sugar baron, returns home to the islands after his heroic military service in World War One. His angry grandmother confronts him because his half Hawaiian mother's death bed note suggests his father was Hawaiian, not her sugar planter son. When she is brutally murdered Grant is arrested. Released from jail, Grant knows he must clear his name quickly because of public pressure. More brutal murders of the elite occur, each with the same note from "Cult of Ku". His frantic search leads him to the Chinese criminal underworld, Hawaiian martial arts lessons, plantation labor union violence, the Bishop Museum, and a unique Honolulu detective who carries a whip. Grant comes close to capturing the killer and races his car down city streets to catch him. A famous Hawaiian swimmer and surfer befriends him. Grant finds relief from tension as he learns to enjoy the ocean sports with native Hawaiians who were not included in his social world. When a navy officer suggests that Japan is preparing for war, Grant finds himself recruited to be a spy. Grant worries about losing his sugar baron father's love because of the clouded ancestry. Torn by his love for his father but discovering the brutal side of the sugar industry toward its workers, Grant struggles for an answer. A Euro-Asian scholar helps him in his search. As Grant races to try to prevent another murder, he accosts the killer at a seaside heiau (temple) where they battle amidst crashing waves. The discovery of the identity of the killer and his motive stun him.