Those who followed Norah Lofts' fictional version of the grandly tragic career of Queen Matilda of Denmark (The Lost Queen, 1968) will find here more sense and substance but no less drama. Caroline Matilda, sister of George III of England, was married off at 15 to young, wildly unstable Christian of Denmark. As the King, whom Walpole described as ""an insipid boy,"" became more abusive and eccentric, an extraordinary German doctor, Johann Struensee, the King's physician, began to impress his intelligence and personality upon the royal pair and, incredibly, Denmark in general. He calmed the King, became the Queen's lover, and inaugurated a series of political and social reforms as the considerable power behind a throne held by children. But inevitably a coup took place -- Sturensee was imprisoned and executed; Matilda was exiled, deprived of her two children (one by her lover), and, after being drawn into a plot to regain her throne, died at 23. Ms. Chapman marks down Matilda several points from Miss Lofts' tender victim -- childlike, she had no training for political manipulation and lacked ""the faintest sense of public responsibility."" Struensee was an impatient, non-materialistic ""worker"" interested in cures and diagnoses rather than the dangerously capricious human nature operating in high places. Like Matilda he tended to overlook portents obvious to everyone else: "". . . glancing down at friends and enemies (they) saw them through the wrong end of a telescope -- diminished and negligible."" The account of the doctor's ""conversion"" before his death is plausible and convincing. Not primarily a scholarly investigation, but an insightful, engrossing recreation of that incredible menage -- a mad King, a charming but inept queen, and a gifted man whose vision did not encompass his own salvation.