The murder of a Chinese radical draws the attention of Five-O, who begin to suspect a Chinese spy ring might be responsible. They come to believe Dr. Paul Farrar might be a member of this ring. McGarrett places undercover policewoman Joyce Bennett in Farrar's class, but what he doesn't know is that Farrar reports to the sinister Wo Fat, who once taught the psychologist advanced hypnotic techniques! Wo Fat has informants everywhere, and they soon reveal Bennett's real identity, allowing Farrar and his skills to turn her into a lethal operative aimed directly at the chief of Five-O.