The detectives of Britain's Department Z look into a murder at an exclusive club.   Sir Basil Riordon is mysterious, frightening, and immensely wealthy. But is he also a killer? As head of England's elite secret service, Department Z, Gordon Craigie has to find out, but it won't be an easy investigation with so many important people involved.   A member of an exclusive London club has been poisoned, and Riordon or his son, Marcus, may be involved. Craigie assigns his best agent, Devenish, to find the truth. But what Devenish finds isn't a clear-cut case of homicide, but a mad masquerade of murder, larceny, and deceit of the highest order. And he and Craigie may be in over their heads . . .