In 1653, Cromwell sent Luke Tremayne and his deputy Harry Lloyd to Paris to negotiate secretly with the exiled king Charles II. After a serious mishap to Luke, Harry is forced to complete the mission alone. Luke is nevertheless gainfully employed by a wealthy French aristocrat, the Marquis des Anges, to investigate the murder of his first wife and the attempted murder of his second. Harry assists the English courtiers to solve the murder of two young ladies-in-waiting to the Queen Mother Henriette Marie and together with a Royalist peer is falsely imprisoned and tortured. He escapes and after many life-threatening adventures is resued on the orders of France's chief minister, Cardinal Mazarin. Meanwhile Luke's investigations are complicated by a feisty abbess, hysterical nuns, a Canadian adventurer, a rampaging bear, and a mysterious treasure of English Catholic gold and silver-investigations that provoke a series of fatal bombings. Harry's determination to find and exact revenge on a renegade French aristocrat responsible for his torture leads him eventually to the French chateau where Luke is pursuing his villains.