Leonard Parks is an agent of the U.K. Inland Revenue who goes missing while conducting a series of interviews with wealthy British expatriates living in Europe. Are these individuals merely concerned with evading taxes, or do they more sinister backgrounds and motives? Inspector George Mason, ably assisted by Detective Sergeant Alison Aubrey, is assigned by Scotland Yard to help discover what happened to the tax official. Their investigations take them to several European cities, where they eventually become involved with members of a secret society who are determined to frustrate their efforts. Will the two detectives succeed in outwitting the opposition, to determine the curious fate of Leonard Parks? Will they find him alive, or dead?